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.go

@@ -14,12 +14,17 @@
 package source
 
 import (
+	"fmt"
 	"io"
 	"os"
 	"path/filepath"
 	"strings"
 	"sync"
 
+	"github.com/spf13/afero"
+
+	"github.com/gohugoio/hugo/hugofs"
+
 	"github.com/gohugoio/hugo/helpers"
 )
 
@@ -86,7 +91,10 @@ type FileInfo struct {
 
 	// Absolute filename to the file on disk.
 	filename string
-	fi       os.FileInfo
+
+	sp *SourceSpec
+
+	fi os.FileInfo
 
 	// Derived from filename
 	ext  string // Extension without any "."
@@ -104,8 +112,6 @@ type FileInfo struct {
 
 	uniqueID string
 
-	sp *SourceSpec
-
 	lazyInit sync.Once
 }
 
@@ -146,7 +152,6 @@ func (fi *FileInfo) init() {
 	fi.lazyInit.Do(func() {
 		relDir := strings.Trim(fi.relDir, helpers.FilePathSeparator)
 		parts := strings.Split(relDir, helpers.FilePathSeparator)
-
 		var section string
 		if (!fi.isLeafBundle && len(parts) == 1) || len(parts) > 1 {
 			section = parts[0]
@@ -161,6 +166,19 @@ func (fi *FileInfo) init() {
 
 func (sp *SourceSpec) NewFileInfo(baseDir, filename string, isLeafBundle bool, fi os.FileInfo) *FileInfo {
 
+	var lang, translationBaseName, relPath string
+
+	if fp, ok := fi.(hugofs.FilePather); ok {
+		filename = fp.Filename()
+		baseDir = fp.BaseDir()
+		relPath = fp.Path()
+	}
+
+	if fl, ok := fi.(hugofs.LanguageAnnouncer); ok {
+		lang = fl.Lang()
+		translationBaseName = fl.TranslationBaseName()
+	}
+
 	dir, name := filepath.Split(filename)
 	if !strings.HasSuffix(dir, helpers.FilePathSeparator) {
 		dir = dir + helpers.FilePathSeparator
@@ -175,19 +193,20 @@ func (sp *SourceSpec) NewFileInfo(baseDir, filename string, isLeafBundle bool, f
 
 	relDir = strings.TrimPrefix(relDir, helpers.FilePathSeparator)
 
-	relPath := filepath.Join(relDir, name)
+	if relPath == "" {
+		relPath = filepath.Join(relDir, name)
+	}
 
 	ext := strings.ToLower(strings.TrimPrefix(filepath.Ext(name), "."))
 	baseName := helpers.Filename(name)
 
-	lang := strings.TrimPrefix(filepath.Ext(baseName), ".")
-	var translationBaseName string
-
-	if _, ok := sp.Languages[lang]; lang == "" || !ok {
-		lang = sp.DefaultContentLanguage
-		translationBaseName = baseName
-	} else {
-		translationBaseName = helpers.Filename(baseName)
+	if translationBaseName == "" {
+		// This is usyally provided by the filesystem. But this FileInfo is also
+		// created in a standalone context when doing "hugo new". This is
+		// an approximate implementation, which is "good enough" in that case.
+		translationBaseName = strings.TrimSuffix(baseName, ext)
+		fileLangExt := filepath.Ext(translationBaseName)
+		translationBaseName = strings.TrimSuffix(translationBaseName, fileLangExt)
 	}
 
 	f := &FileInfo{
@@ -211,5 +230,27 @@ func (sp *SourceSpec) NewFileInfo(baseDir, filename string, isLeafBundle bool, f
 
 // Open implements ReadableFile.
 func (fi *FileInfo) Open() (io.ReadCloser, error) {
-	return fi.sp.Fs.Source.Open(fi.Filename())
+	f, err := fi.sp.PathSpec.Fs.Source.Open(fi.Filename())
+	return f, err
+}
+
+func printFs(fs afero.Fs, path string, w io.Writer) {
+	if fs == nil {
+		return
+	}
+	afero.Walk(fs, path, func(path string, info os.FileInfo, err error) error {
+
+		if info != nil && !info.IsDir() {
+
+			s := path
+			if lang, ok := info.(hugofs.LanguageAnnouncer); ok {
+				s = s + "\t" + lang.Lang()
+			}
+			if fp, ok := info.(hugofs.FilePather); ok {
+				s = s + "\t" + fp.Filename()
+			}
+			fmt.Fprintln(w, "    ", s)
+		}
+		return nil
+	})
 }