all: Refactor to nonglobal Viper, i18n etc.

This is a final rewrite that removes all the global state in Hugo, which also enables
the use if `t.Parallel` in tests.

Updates #2701
Fixes #3016

hugofs/fs.go

@@ -16,7 +16,7 @@ package hugofs
 
 import (
 	"github.com/spf13/afero"
-	"github.com/spf13/viper"
+	"github.com/spf13/hugo/config"
 )
 
 // Os points to an Os Afero file system.
@@ -39,30 +39,37 @@ type Fs struct {
 
 // NewDefault creates a new Fs with the OS file system
 // as source and destination file systems.
-func NewDefault() *Fs {
+func NewDefault(cfg config.Provider) *Fs {
 	fs := &afero.OsFs{}
-	return newFs(fs)
+	return newFs(fs, cfg)
 }
 
 // NewDefault creates a new Fs with the MemMapFs
 // as source and destination file systems.
 // Useful for testing.
-func NewMem() *Fs {
+func NewMem(cfg config.Provider) *Fs {
 	fs := &afero.MemMapFs{}
-	return newFs(fs)
+	return newFs(fs, cfg)
 }
 
-func newFs(base afero.Fs) *Fs {
+// NewFrom creates a new Fs based on the provided Afero Fs
+// as source and destination file systems.
+// Useful for testing.
+func NewFrom(fs afero.Fs, cfg config.Provider) *Fs {
+	return newFs(fs, cfg)
+}
+
+func newFs(base afero.Fs, cfg config.Provider) *Fs {
 	return &Fs{
 		Source:      base,
 		Destination: base,
 		Os:          &afero.OsFs{},
-		WorkingDir:  getWorkingDirFs(base),
+		WorkingDir:  getWorkingDirFs(base, cfg),
 	}
 }
 
-func getWorkingDirFs(base afero.Fs) *afero.BasePathFs {
-	workingDir := viper.GetString("workingDir")
+func getWorkingDirFs(base afero.Fs, cfg config.Provider) *afero.BasePathFs {
+	workingDir := cfg.GetString("workingDir")
 
 	if workingDir != "" {
 		return afero.NewBasePathFs(afero.NewReadOnlyFs(base), workingDir).(*afero.BasePathFs)