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/site_url_test.go

@@ -119,10 +119,12 @@ Do not go gentle into that good night.
 
 	notUgly := s.getPage(KindPage, "sect1/p1.md")
 	assert.NotNil(notUgly)
+	assert.Equal("sect1", notUgly.Section())
 	assert.Equal("/sect1/p1/", notUgly.RelPermalink())
 
 	ugly := s.getPage(KindPage, "sect2/p2.md")
 	assert.NotNil(ugly)
+	assert.Equal("sect2", ugly.Section())
 	assert.Equal("/sect2/p2.html", ugly.RelPermalink())
 }