Add support for a content dir set per language

A sample config:

```toml
defaultContentLanguage = "en"
defaultContentLanguageInSubdir = true

[Languages]
[Languages.en]
weight = 10
title = "In English"
languageName = "English"
contentDir = "content/english"

[Languages.nn]
weight = 20
title = "På Norsk"
languageName = "Norsk"
contentDir = "content/norwegian"
```

The value of `contentDir` can be any valid path, even absolute path references. The only restriction is that the content dirs cannot overlap.

The content files will be assigned a language by

1. The placement: `content/norwegian/post/my-post.md` will be read as Norwegian content.
2. The filename: `content/english/post/my-post.nn.md` will be read as Norwegian even if it lives in the English content folder.

The content directories will be merged into a big virtual filesystem with one simple rule: The most specific language file will win.
This means that if both `content/norwegian/post/my-post.md` and `content/english/post/my-post.nn.md` exists, they will be considered duplicates and the version inside `content/norwegian` will win.

Note that translations will be automatically assigned by Hugo by the content file's relative placement, so `content/norwegian/post/my-post.md` will be a translation of `content/english/post/my-post.md`.

If this does not work for you, you can connect the translations together by setting a `translationKey` in the content files' front matter.

Fixes #4523
Fixes #4552
Fixes #4553

hugolib/language_content_dir_test.go

@@ -0,0 +1,253 @@
+// Copyright 2018 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hugolib
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+/*
+
+/en/p1.md
+/nn/p1.md
+
+.Readdir
+
+- Name() => p1.en.md, p1.nn.md
+
+.Stat(name)
+
+.Open() --- real file name
+
+
+*/
+
+func TestLanguageContentRoot(t *testing.T) {
+	t.Parallel()
+	assert := require.New(t)
+
+	config := `
+baseURL = "https://example.org/"
+
+defaultContentLanguage = "en"
+defaultContentLanguageInSubdir = true
+
+contentDir = "content/main"
+workingDir = "/my/project"
+
+[Languages]
+[Languages.en]
+weight = 10
+title = "In English"
+languageName = "English"
+
+[Languages.nn]
+weight = 20
+title = "På Norsk"
+languageName = "Norsk"
+# This tells Hugo that all content in this directory is in the Norwegian language.
+# It does not have to have the "my-page.nn.md" format. It can, but that is optional.
+contentDir = "content/norsk"
+
+[Languages.sv]
+weight = 30
+title = "På Svenska"
+languageName = "Svensk"
+contentDir = "content/svensk"
+`
+
+	pageTemplate := `
+---
+title: %s
+slug: %s
+weight: %d
+---
+
+Content.
+
+`
+
+	pageBundleTemplate := `
+---
+title: %s
+weight: %d
+---
+
+Content.
+
+`
+	var contentFiles []string
+	section := "sect"
+
+	var contentRoot = func(lang string) string {
+		contentRoot := "content/main"
+
+		switch lang {
+		case "nn":
+			contentRoot = "content/norsk"
+		case "sv":
+			contentRoot = "content/svensk"
+		}
+		return contentRoot + "/" + section
+	}
+
+	for _, lang := range []string{"en", "nn", "sv"} {
+		for j := 1; j <= 10; j++ {
+			if (lang == "nn" || lang == "en") && j%4 == 0 {
+				// Skip 4 and 8 for nn
+				// We also skip it for en, but that is added to the Swedish directory below.
+				continue
+			}
+
+			if lang == "sv" && j%5 == 0 {
+				// Skip 5 and 10 for sv
+				continue
+			}
+
+			base := fmt.Sprintf("p-%s-%d", lang, j)
+			slug := fmt.Sprintf("%s", base)
+			langID := ""
+
+			if lang == "sv" && j%4 == 0 {
+				// Put an English page in the Swedish content dir.
+				langID = ".en"
+			}
+
+			if lang == "en" && j == 8 {
+				// This should win over the sv variant above.
+				langID = ".en"
+			}
+
+			slug += langID
+
+			contentRoot := contentRoot(lang)
+
+			filename := filepath.Join(contentRoot, fmt.Sprintf("page%d%s.md", j, langID))
+			contentFiles = append(contentFiles, filename, fmt.Sprintf(pageTemplate, slug, slug, j))
+		}
+	}
+
+	// Put common translations in all of them
+	for i, lang := range []string{"en", "nn", "sv"} {
+		contentRoot := contentRoot(lang)
+
+		slug := fmt.Sprintf("common_%s", lang)
+
+		filename := filepath.Join(contentRoot, "common.md")
+		contentFiles = append(contentFiles, filename, fmt.Sprintf(pageTemplate, slug, slug, 100+i))
+
+		for j, lang2 := range []string{"en", "nn", "sv"} {
+			filename := filepath.Join(contentRoot, fmt.Sprintf("translated_all.%s.md", lang2))
+			langSlug := slug + "_translated_all_" + lang2
+			contentFiles = append(contentFiles, filename, fmt.Sprintf(pageTemplate, langSlug, langSlug, 200+i+j))
+		}
+
+		for j, lang2 := range []string{"sv", "nn"} {
+			if lang == "en" {
+				continue
+			}
+			filename := filepath.Join(contentRoot, fmt.Sprintf("translated_some.%s.md", lang2))
+			langSlug := slug + "_translated_some_" + lang2
+			contentFiles = append(contentFiles, filename, fmt.Sprintf(pageTemplate, langSlug, langSlug, 300+i+j))
+		}
+	}
+
+	// Add a bundle with some images
+	for i, lang := range []string{"en", "nn", "sv"} {
+		contentRoot := contentRoot(lang)
+		slug := fmt.Sprintf("bundle_%s", lang)
+		filename := filepath.Join(contentRoot, "mybundle", "index.md")
+		contentFiles = append(contentFiles, filename, fmt.Sprintf(pageBundleTemplate, slug, 400+i))
+		if lang == "en" {
+			imageFilename := filepath.Join(contentRoot, "mybundle", "logo.png")
+			contentFiles = append(contentFiles, imageFilename, "PNG Data")
+		}
+		imageFilename := filepath.Join(contentRoot, "mybundle", "featured.png")
+		contentFiles = append(contentFiles, imageFilename, fmt.Sprintf("PNG Data for %s", lang))
+
+		// Add some bundled pages
+		contentFiles = append(contentFiles, filepath.Join(contentRoot, "mybundle", "p1.md"), fmt.Sprintf(pageBundleTemplate, slug, 401+i))
+		contentFiles = append(contentFiles, filepath.Join(contentRoot, "mybundle", "sub", "p1.md"), fmt.Sprintf(pageBundleTemplate, slug, 402+i))
+
+	}
+
+	b := newTestSitesBuilder(t)
+	b.WithWorkingDir("/my/project").WithConfigFile("toml", config).WithContent(contentFiles...).CreateSites()
+
+	_ = os.Stdout
+	//printFs(b.H.BaseFs.ContentFs, "/", os.Stdout)
+
+	b.Build(BuildCfg{})
+
+	assert.Equal(3, len(b.H.Sites))
+
+	enSite := b.H.Sites[0]
+	nnSite := b.H.Sites[1]
+	svSite := b.H.Sites[2]
+
+	//dumpPages(nnSite.RegularPages...)
+	assert.Equal(12, len(nnSite.RegularPages))
+	assert.Equal(13, len(enSite.RegularPages))
+
+	assert.Equal(10, len(svSite.RegularPages))
+
+	for i, p := range enSite.RegularPages {
+		j := i + 1
+		msg := fmt.Sprintf("Test %d", j)
+		assert.Equal("en", p.Lang(), msg)
+		assert.Equal("sect", p.Section())
+		if j < 9 {
+			if j%4 == 0 {
+				assert.Contains(p.Title(), fmt.Sprintf("p-sv-%d.en", i+1), msg)
+			} else {
+				assert.Contains(p.Title(), "p-en", msg)
+			}
+		}
+	}
+
+	// Check bundles
+	bundleEn := enSite.RegularPages[len(enSite.RegularPages)-1]
+	bundleNn := nnSite.RegularPages[len(nnSite.RegularPages)-1]
+	bundleSv := svSite.RegularPages[len(svSite.RegularPages)-1]
+
+	assert.Equal("/en/sect/mybundle/", bundleEn.RelPermalink())
+	assert.Equal("/sv/sect/mybundle/", bundleSv.RelPermalink())
+
+	assert.Equal(4, len(bundleEn.Resources))
+	assert.Equal(4, len(bundleNn.Resources))
+	assert.Equal(4, len(bundleSv.Resources))
+
+	assert.Equal("/en/sect/mybundle/logo.png", bundleEn.Resources.GetMatch("logo*").RelPermalink())
+	assert.Equal("/nn/sect/mybundle/logo.png", bundleNn.Resources.GetMatch("logo*").RelPermalink())
+	assert.Equal("/sv/sect/mybundle/logo.png", bundleSv.Resources.GetMatch("logo*").RelPermalink())
+
+	b.AssertFileContent("/my/project/public/sv/sect/mybundle/featured.png", "PNG Data for sv")
+	b.AssertFileContent("/my/project/public/nn/sect/mybundle/featured.png", "PNG Data for nn")
+	b.AssertFileContent("/my/project/public/en/sect/mybundle/featured.png", "PNG Data for en")
+	b.AssertFileContent("/my/project/public/en/sect/mybundle/logo.png", "PNG Data")
+	b.AssertFileContent("/my/project/public/sv/sect/mybundle/logo.png", "PNG Data")
+	b.AssertFileContent("/my/project/public/nn/sect/mybundle/logo.png", "PNG Data")
+
+	nnSect := nnSite.getPage(KindSection, "sect")
+	assert.NotNil(nnSect)
+	assert.Equal(12, len(nnSect.Pages))
+	nnHome, _ := nnSite.Info.Home()
+	assert.Equal("/nn/", nnHome.RelPermalink())
+
+}