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

tpl/data/data_test.go

@@ -21,7 +21,6 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/spf13/viper"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
@@ -29,7 +28,7 @@ import (
 func TestGetCSV(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	ns := newTestNs()
 
 	for i, test := range []struct {
 		sep     string
@@ -123,7 +122,7 @@ func TestGetCSV(t *testing.T) {
 func TestGetJSON(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	ns := newTestNs()
 
 	for i, test := range []struct {
 		url     string