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

source/fileInfo_test.go

@@ -17,6 +17,12 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/gohugoio/hugo/helpers"
+
+	"github.com/spf13/viper"
+
+	"github.com/gohugoio/hugo/hugofs"
+	"github.com/spf13/afero"
 	"github.com/stretchr/testify/require"
 )
 
@@ -35,6 +41,8 @@ func TestFileInfo(t *testing.T) {
 			assert.Equal(filepath.FromSlash("b/"), f.Dir())
 			assert.Equal(filepath.FromSlash("b/page.md"), f.Path())
 			assert.Equal("b", f.Section())
+			assert.Equal(filepath.FromSlash("page"), f.TranslationBaseName())
+			assert.Equal(filepath.FromSlash("page"), f.BaseFileName())
 
 		}},
 		{filepath.FromSlash("/a/"), filepath.FromSlash("/a/b/c/d/page.md"), func(f *FileInfo) {
@@ -47,3 +55,39 @@ func TestFileInfo(t *testing.T) {
 	}
 
 }
+
+func TestFileInfoLanguage(t *testing.T) {
+	assert := require.New(t)
+	langs := map[string]bool{
+		"sv": true,
+		"en": true,
+	}
+
+	m := afero.NewMemMapFs()
+	lfs := hugofs.NewLanguageFs("sv", langs, m)
+	v := viper.New()
+	v.Set("contentDir", "content")
+
+	fs := hugofs.NewFrom(m, v)
+
+	ps, err := helpers.NewPathSpec(fs, v)
+	assert.NoError(err)
+	s := SourceSpec{Fs: lfs, PathSpec: ps}
+	s.Languages = map[string]interface{}{
+		"en": true,
+	}
+
+	err = afero.WriteFile(lfs, "page.md", []byte("abc"), 0777)
+	assert.NoError(err)
+	err = afero.WriteFile(lfs, "page.en.md", []byte("abc"), 0777)
+	assert.NoError(err)
+
+	sv, _ := lfs.Stat("page.md")
+	en, _ := lfs.Stat("page.en.md")
+
+	fiSv := s.NewFileInfo("", "page.md", false, sv)
+	fiEn := s.NewFileInfo("", "page.en.md", false, en)
+
+	assert.Equal("sv", fiSv.Lang())
+	assert.Equal("en", fiEn.Lang())
+}