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/transform/transform_test.go

@@ -26,7 +26,7 @@ import (
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/langs"
-	"github.com/spf13/viper"
+	
 )
 
 type tstNoStringer struct{}
@@ -35,7 +35,7 @@ func TestEmojify(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 
-	v := viper.New()
+	v := config.New()
 	ns := New(newDeps(v))
 
 	for _, test := range []struct {
@@ -64,7 +64,7 @@ func TestHighlight(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 
@@ -96,7 +96,7 @@ func TestHTMLEscape(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 
@@ -126,7 +126,7 @@ func TestHTMLUnescape(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 
@@ -156,7 +156,7 @@ func TestMarkdownify(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 
@@ -185,7 +185,7 @@ func TestMarkdownify(t *testing.T) {
 func TestMarkdownifyBlocksOfText(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 
@@ -211,7 +211,7 @@ func TestPlainify(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 
-	v := viper.New()
+	v := config.New()
 	ns := New(newDeps(v))
 
 	for _, test := range []struct {