Misc config loading fixes

The main motivation behind this is simplicity and correctnes, but the new small config library is also faster:

```
BenchmarkDefaultConfigProvider/Viper-16         	  252418	      4546 ns/op	    2720 B/op	      30 allocs/op
BenchmarkDefaultConfigProvider/Custom-16        	  450756	      2651 ns/op	    1008 B/op	       6 allocs/op
```

Fixes #8633
Fixes #8618
Fixes #8630
Updates #8591
Closes #6680
Closes #5192

tpl/data/init_test.go

@@ -17,10 +17,10 @@ import (
 	"testing"
 
 	qt "github.com/frankban/quicktest"
+	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/htesting/hqt"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/tpl/internal"
-	"github.com/spf13/viper"
 )
 
 func TestInit(t *testing.T) {
@@ -28,7 +28,7 @@ func TestInit(t *testing.T) {
 	var found bool
 	var ns *internal.TemplateFuncsNamespace
 
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	langs.LoadLanguageSettings(v, nil)
 

tpl/data/resources_test.go

@@ -34,12 +34,12 @@ import (
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/spf13/afero"
-	"github.com/spf13/viper"
+	
 )
 
 func TestScpGetLocal(t *testing.T) {
 	t.Parallel()
-	v := viper.New()
+	v := config.New()
 	fs := hugofs.NewMem(v)
 	ps := helpers.FilePathSeparator
 
@@ -144,7 +144,7 @@ func TestScpGetRemoteParallel(t *testing.T) {
 	c.Assert(err, qt.IsNil)
 
 	for _, ignoreCache := range []bool{false} {
-		cfg := viper.New()
+		cfg := config.New()
 		cfg.Set("ignoreCache", ignoreCache)
 		cfg.Set("contentDir", "content")
 
@@ -223,7 +223,7 @@ func newDeps(cfg config.Provider) *deps.Deps {
 }
 
 func newTestNs() *Namespace {
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	return New(newDeps(v))
 }