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/fileInfo_test.go

@@ -1,61 +0,0 @@
-// Copyright 2017-present 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 (
-	"testing"
-
-	"path/filepath"
-
-	"github.com/gohugoio/hugo/source"
-	"github.com/stretchr/testify/require"
-)
-
-func TestBundleFileInfo(t *testing.T) {
-	t.Parallel()
-
-	assert := require.New(t)
-	cfg, fs := newTestBundleSourcesMultilingual(t)
-	sourceSpec := source.NewSourceSpec(cfg, fs)
-
-	for _, this := range []struct {
-		filename string
-		check    func(f *fileInfo)
-	}{
-		{"/path/to/file.md", func(fi *fileInfo) {
-			assert.Equal("md", fi.Ext())
-			assert.Equal("en", fi.Lang())
-			assert.False(fi.isOwner())
-			assert.True(fi.isContentFile())
-		}},
-		{"/path/to/file.JPG", func(fi *fileInfo) {
-			assert.Equal("jpg", fi.Ext())
-			assert.False(fi.isContentFile())
-		}},
-		{"/path/to/file.nn.png", func(fi *fileInfo) {
-			assert.Equal("png", fi.Ext())
-			assert.Equal("nn", fi.Lang())
-			assert.Equal("file", fi.TranslationBaseName())
-			assert.False(fi.isContentFile())
-		}},
-	} {
-		fi := newFileInfo(
-			sourceSpec,
-			filepath.FromSlash("/work/base"),
-			filepath.FromSlash(this.filename),
-			nil, bundleNot)
-		this.check(fi)
-	}
-
-}