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

3 files changed, 18 insertions, 17 deletions

diff --git a/tpl/transform/remarshal_test.go b/tpl/transform/remarshal_test.go
index eb5f4253c..9f3e05e61 100644
--- a/tpl/transform/remarshal_test.go
+++ b/tpl/transform/remarshal_test.go
@@ -16,16 +16,16 @@ package transform
 import (
 	"testing"
 
+	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/htesting"
 
 	qt "github.com/frankban/quicktest"
-	"github.com/spf13/viper"
 )
 
 func TestRemarshal(t *testing.T) {
 	t.Parallel()
 
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 	c := qt.New(t)
@@ -112,7 +112,7 @@ title: Test Metadata
 func TestRemarshalComments(t *testing.T) {
 	t.Parallel()
 
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 
@@ -158,7 +158,7 @@ func TestTestRemarshalError(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
 
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 
@@ -172,7 +172,7 @@ func TestTestRemarshalError(t *testing.T) {
 func TestTestRemarshalMapInput(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
-	v := viper.New()
+	v := config.New()
 	v.Set("contentDir", "content")
 	ns := New(newDeps(v))
 
diff --git a/tpl/transform/transform_test.go b/tpl/transform/transform_test.go
index b98d82d27..1dbf97f98 100644
--- a/tpl/transform/transform_test.go
+++ b/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 {
diff --git a/tpl/transform/unmarshal_test.go b/tpl/transform/unmarshal_test.go
index ec81c316a..85e3610d1 100644
--- a/tpl/transform/unmarshal_test.go
+++ b/tpl/transform/unmarshal_test.go
@@ -19,13 +19,14 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/gohugoio/hugo/config"
+
 	"github.com/gohugoio/hugo/common/hugio"
 	"github.com/gohugoio/hugo/resources/resource"
 
 	"github.com/gohugoio/hugo/media"
 
 	qt "github.com/frankban/quicktest"
-	"github.com/spf13/viper"
 )
 
 const (
@@ -79,7 +80,7 @@ func (t testContentResource) Key() string {
 }
 
 func TestUnmarshal(t *testing.T) {
-	v := viper.New()
+	v := config.New()
 	ns := New(newDeps(v))
 	c := qt.New(t)
 
@@ -173,7 +174,7 @@ a;b;c`, mime: media.CSVType}, map[string]interface{}{"DElimiter": ";", "Comment"
 }
 
 func BenchmarkUnmarshalString(b *testing.B) {
-	v := viper.New()
+	v := config.New()
 	ns := New(newDeps(v))
 
 	const numJsons = 100
@@ -196,7 +197,7 @@ func BenchmarkUnmarshalString(b *testing.B) {
 }
 
 func BenchmarkUnmarshalResource(b *testing.B) {
-	v := viper.New()
+	v := config.New()
 	ns := New(newDeps(v))
 
 	const numJsons = 100