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

helpers/path_test.go

@@ -57,8 +57,10 @@ func TestMakePath(t *testing.T) {
 
 	for _, test := range tests {
 		v := viper.New()
-		l := NewDefaultLanguage(v)
+		v.Set("contentDir", "content")
 		v.Set("removePathAccents", test.removeAccents)
+
+		l := NewDefaultLanguage(v)
 		p, err := NewPathSpec(hugofs.NewMem(v), l)
 		require.NoError(t, err)
 
@@ -71,6 +73,8 @@ func TestMakePath(t *testing.T) {
 
 func TestMakePathSanitized(t *testing.T) {
 	v := viper.New()
+	v.Set("contentDir", "content")
+
 	l := NewDefaultLanguage(v)
 	p, _ := NewPathSpec(hugofs.NewMem(v), l)
 
@@ -98,6 +102,7 @@ func TestMakePathSanitizedDisablePathToLower(t *testing.T) {
 	v := viper.New()
 
 	v.Set("disablePathToLower", true)
+	v.Set("contentDir", "content")
 
 	l := NewDefaultLanguage(v)
 	p, _ := NewPathSpec(hugofs.NewMem(v), l)