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

1 files changed, 17 insertions, 2 deletions

diff --git a/hugolib/fileInfo.go b/hugolib/fileInfo.go
index f819f6bfc..90cf91377 100644
--- a/hugolib/fileInfo.go
+++ b/hugolib/fileInfo.go
@@ -14,7 +14,6 @@
 package hugolib
 
 import (
-	"os"
 	"strings"
 
 	"github.com/gohugoio/hugo/helpers"
@@ -25,11 +24,22 @@ import (
 var (
 	_ source.File         = (*fileInfo)(nil)
 	_ source.ReadableFile = (*fileInfo)(nil)
+	_ pathLangFile        = (*fileInfo)(nil)
 )
 
+// A partial interface to prevent ambigous compiler error.
+type basePather interface {
+	Filename() string
+	RealName() string
+	BaseDir() string
+}
+
 type fileInfo struct {
 	bundleTp bundleDirType
+
 	source.ReadableFile
+	basePather
+
 	overriddenLang string
 
 	// Set if the content language for this file is disabled.
@@ -43,6 +53,10 @@ func (fi *fileInfo) Lang() string {
 	return fi.ReadableFile.Lang()
 }
 
+func (fi *fileInfo) Filename() string {
+	return fi.basePather.Filename()
+}
+
 func (fi *fileInfo) isOwner() bool {
 	return fi.bundleTp > bundleNot
 }
@@ -55,12 +69,13 @@ func (fi *fileInfo) isContentFile() bool {
 	return contentFileExtensionsSet[fi.Ext()]
 }
 
-func newFileInfo(sp *source.SourceSpec, baseDir, filename string, fi os.FileInfo, tp bundleDirType) *fileInfo {
+func newFileInfo(sp *source.SourceSpec, baseDir, filename string, fi pathLangFileFi, tp bundleDirType) *fileInfo {
 
 	baseFi := sp.NewFileInfo(baseDir, filename, tp == bundleLeaf, fi)
 	f := &fileInfo{
 		bundleTp:     tp,
 		ReadableFile: baseFi,
+		basePather:   fi,
 	}
 
 	lang := f.Lang()