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

8 files changed, 73 insertions, 21 deletions

diff --git a/tpl/collections/collections_test.go b/tpl/collections/collections_test.go
index f35e29459..68e7c59d6 100644
--- a/tpl/collections/collections_test.go
+++ b/tpl/collections/collections_test.go
@@ -369,7 +369,7 @@ func TestIntersect(t *testing.T) {
 func TestIsSet(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	ns := newTestNs()
 
 	for i, test := range []struct {
 		a      interface{}
@@ -787,3 +787,9 @@ func newDeps(cfg config.Provider) *deps.Deps {
 		Log:         jww.NewNotepad(jww.LevelError, jww.LevelError, os.Stdout, ioutil.Discard, "", log.Ldate|log.Ltime),
 	}
 }
+
+func newTestNs() *Namespace {
+	v := viper.New()
+	v.Set("contentDir", "content")
+	return New(newDeps(v))
+}
diff --git a/tpl/data/data_test.go b/tpl/data/data_test.go
index bcdddc9f4..9b21dc8aa 100644
--- a/tpl/data/data_test.go
+++ b/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
diff --git a/tpl/data/resources_test.go b/tpl/data/resources_test.go
index f0b027955..79e9b3907 100644
--- a/tpl/data/resources_test.go
+++ b/tpl/data/resources_test.go
@@ -127,7 +127,7 @@ func TestScpGetRemote(t *testing.T) {
 func TestScpGetRemoteParallel(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	ns := newTestNs()
 
 	content := []byte(`T€st Content 123`)
 	srv, cl := getTestServer(func(w http.ResponseWriter, r *http.Request) {
@@ -176,3 +176,9 @@ func newDeps(cfg config.Provider) *deps.Deps {
 		ContentSpec: cs,
 	}
 }
+
+func newTestNs() *Namespace {
+	v := viper.New()
+	v.Set("contentDir", "content")
+	return New(newDeps(v))
+}
diff --git a/tpl/os/os.go b/tpl/os/os.go
index 02faa2809..f005bd4a9 100644
--- a/tpl/os/os.go
+++ b/tpl/os/os.go
@@ -25,14 +25,29 @@ import (
 
 // New returns a new instance of the os-namespaced template functions.
 func New(deps *deps.Deps) *Namespace {
+
+	// Since Hugo 0.38 we can have multiple content dirs. This can make it hard to
+	// reason about where the file is placed relative to the project root.
+	// To make the {{ readFile .Filename }} variant just work, we create a composite
+	// filesystem that first checks the work dir fs and then the content fs.
+	var rfs afero.Fs
+	if deps.Fs != nil {
+		rfs = deps.Fs.WorkingDir
+		if deps.PathSpec != nil && deps.PathSpec.BaseFs != nil {
+			rfs = afero.NewReadOnlyFs(afero.NewCopyOnWriteFs(deps.PathSpec.BaseFs.ContentFs, deps.Fs.WorkingDir))
+		}
+	}
+
 	return &Namespace{
-		deps: deps,
+		readFileFs: rfs,
+		deps:       deps,
 	}
 }
 
 // Namespace provides template functions for the "os" namespace.
 type Namespace struct {
-	deps *deps.Deps
+	readFileFs afero.Fs
+	deps       *deps.Deps
 }
 
 // Getenv retrieves the value of the environment variable named by the key.
@@ -46,10 +61,10 @@ func (ns *Namespace) Getenv(key interface{}) (string, error) {
 	return _os.Getenv(skey), nil
 }
 
-// readFile reads the file named by filename relative to the given basepath
+// readFile reads the file named by filename in the given filesystem
 // and returns the contents as a string.
 // There is a upper size limit set at 1 megabytes.
-func readFile(fs *afero.BasePathFs, filename string) (string, error) {
+func readFile(fs afero.Fs, filename string) (string, error) {
 	if filename == "" {
 		return "", errors.New("readFile needs a filename")
 	}
@@ -79,7 +94,7 @@ func (ns *Namespace) ReadFile(i interface{}) (string, error) {
 		return "", err
 	}
 
-	return readFile(ns.deps.Fs.WorkingDir, s)
+	return readFile(ns.readFileFs, s)
 }
 
 // ReadDir lists the directory contents relative to the configured WorkingDir.
diff --git a/tpl/tplimpl/template_funcs_test.go b/tpl/tplimpl/template_funcs_test.go
index b8390c5b9..72842d308 100644
--- a/tpl/tplimpl/template_funcs_test.go
+++ b/tpl/tplimpl/template_funcs_test.go
@@ -65,6 +65,7 @@ func TestTemplateFuncsExamples(t *testing.T) {
 
 	v.Set("workingDir", workingDir)
 	v.Set("multilingual", true)
+	v.Set("contentDir", "content")
 	v.Set("baseURL", "http://mysite.com/hugo/")
 	v.Set("CurrentContentLanguage", helpers.NewLanguage("en", v))
 
@@ -125,7 +126,10 @@ func TestPartialCached(t *testing.T) {
 	var data struct {
 	}
 
-	config := newDepsConfig(viper.New())
+	v := viper.New()
+	v.Set("contentDir", "content")
+
+	config := newDepsConfig(v)
 
 	config.WithTemplate = func(templ tpl.TemplateHandler) error {
 		err := templ.AddTemplate("partials/"+name, partial)
diff --git a/tpl/tplimpl/template_test.go b/tpl/tplimpl/template_test.go
index b94848394..78682e9ab 100644
--- a/tpl/tplimpl/template_test.go
+++ b/tpl/tplimpl/template_test.go
@@ -35,6 +35,7 @@ func TestHTMLEscape(t *testing.T) {
 		"other": "<h1>Hi!</h1>",
 	}
 	v := viper.New()
+	v.Set("contentDir", "content")
 	fs := hugofs.NewMem(v)
 
 	//afero.WriteFile(fs.Source, filepath.Join(workingDir, "README.txt"), []byte("Hugo Rocks!"), 0755)
diff --git a/tpl/transform/remarshal_test.go b/tpl/transform/remarshal_test.go
index 5e3ed8b34..07c51c3b0 100644
--- a/tpl/transform/remarshal_test.go
+++ b/tpl/transform/remarshal_test.go
@@ -25,7 +25,9 @@ import (
 func TestRemarshal(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 	assert := require.New(t)
 
 	tomlExample := `title = "Test Metadata"
@@ -111,7 +113,10 @@ title: Test Metadata
 func TestRemarshalComments(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
+
 	assert := require.New(t)
 
 	input := `
@@ -153,7 +158,9 @@ Hugo = "Rules"
 func TestTestRemarshalError(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 	assert := require.New(t)
 
 	_, err := ns.Remarshal("asdf", "asdf")
diff --git a/tpl/transform/transform_test.go b/tpl/transform/transform_test.go
index 195a0f15c..ab3beb804 100644
--- a/tpl/transform/transform_test.go
+++ b/tpl/transform/transform_test.go
@@ -32,7 +32,9 @@ type tstNoStringer struct{}
 func TestEmojify(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -60,7 +62,9 @@ func TestEmojify(t *testing.T) {
 func TestHighlight(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -90,7 +94,9 @@ func TestHighlight(t *testing.T) {
 func TestHTMLEscape(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -118,7 +124,9 @@ func TestHTMLEscape(t *testing.T) {
 func TestHTMLUnescape(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -146,7 +154,9 @@ func TestHTMLUnescape(t *testing.T) {
 func TestMarkdownify(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}
@@ -176,7 +186,9 @@ func TestMarkdownifyBlocksOfText(t *testing.T) {
 
 	assert := require.New(t)
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	text := `
 #First 
@@ -201,7 +213,9 @@ And then some.
 func TestPlainify(t *testing.T) {
 	t.Parallel()
 
-	ns := New(newDeps(viper.New()))
+	v := viper.New()
+	v.Set("contentDir", "content")
+	ns := New(newDeps(v))
 
 	for i, test := range []struct {
 		s      interface{}