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

resources/page/pagemeta/page_frontmatter_test.go

@@ -18,8 +18,9 @@ import (
 	"testing"
 	"time"
 
+	"github.com/gohugoio/hugo/config"
+
 	"github.com/gohugoio/hugo/resources/resource"
-	"github.com/spf13/viper"
 
 	qt "github.com/frankban/quicktest"
 )
@@ -72,7 +73,7 @@ func newTestFd() *FrontMatterDescriptor {
 func TestFrontMatterNewConfig(t *testing.T) {
 	c := qt.New(t)
 
-	cfg := viper.New()
+	cfg := config.New()
 
 	cfg.Set("frontmatter", map[string]interface{}{
 		"date":        []string{"publishDate", "LastMod"},
@@ -89,7 +90,7 @@ func TestFrontMatterNewConfig(t *testing.T) {
 	c.Assert(fc.publishDate, qt.DeepEquals, []string{"date"})
 
 	// Default
-	cfg = viper.New()
+	cfg = config.New()
 	fc, err = newFrontmatterConfig(cfg)
 	c.Assert(err, qt.IsNil)
 	c.Assert(fc.date, qt.DeepEquals, []string{"date", "publishdate", "pubdate", "published", "lastmod", "modified"})
@@ -117,7 +118,7 @@ func TestFrontMatterDatesHandlers(t *testing.T) {
 
 	for _, handlerID := range []string{":filename", ":fileModTime", ":git"} {
 
-		cfg := viper.New()
+		cfg := config.New()
 
 		cfg.Set("frontmatter", map[string]interface{}{
 			"date": []string{handlerID, "date"},
@@ -157,7 +158,7 @@ func TestFrontMatterDatesCustomConfig(t *testing.T) {
 
 	c := qt.New(t)
 
-	cfg := viper.New()
+	cfg := config.New()
 	cfg.Set("frontmatter", map[string]interface{}{
 		"date":        []string{"mydate"},
 		"lastmod":     []string{"publishdate"},
@@ -204,7 +205,7 @@ func TestFrontMatterDatesDefaultKeyword(t *testing.T) {
 
 	c := qt.New(t)
 
-	cfg := viper.New()
+	cfg := config.New()
 
 	cfg.Set("frontmatter", map[string]interface{}{
 		"date":        []string{"mydate", ":default"},

resources/page/pagination_test.go

@@ -18,7 +18,7 @@ import (
 	"html/template"
 	"testing"
 
-	"github.com/spf13/viper"
+	"github.com/gohugoio/hugo/config"
 
 	qt "github.com/frankban/quicktest"
 	"github.com/gohugoio/hugo/output"
@@ -196,7 +196,7 @@ func doTestPagerNoPages(t *testing.T, paginator *Paginator) {
 func TestPaginationURLFactory(t *testing.T) {
 	t.Parallel()
 	c := qt.New(t)
-	cfg := viper.New()
+	cfg := config.New()
 	cfg.Set("paginatePath", "zoo")
 
 	for _, uglyURLs := range []bool{false, true} {

resources/page/testhelpers_test.go

@@ -29,7 +29,7 @@ import (
 	"github.com/bep/gitmap"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/resources/resource"
-	"github.com/spf13/viper"
+	
 
 	"github.com/gohugoio/hugo/navigation"
 
@@ -69,7 +69,7 @@ func newTestPageWithFile(filename string) *testPage {
 }
 
 func newTestPathSpec() *helpers.PathSpec {
-	return newTestPathSpecFor(viper.New())
+	return newTestPathSpecFor(config.New())
 }
 
 func newTestPathSpecFor(cfg config.Provider) *helpers.PathSpec {

resources/resource_metadata.go

@@ -130,7 +130,7 @@ func AssignMetadata(metadata []map[string]interface{}, resources ...resource.Res
 				if found {
 					m := maps.ToStringMap(params)
 					// Needed for case insensitive fetching of params values
-					maps.ToLower(m)
+					maps.PrepareParams(m)
 					ma.updateParams(m)
 				}
 			}

resources/resource_transformers/htesting/testhelpers.go

@@ -17,17 +17,17 @@ import (
 	"path/filepath"
 
 	"github.com/gohugoio/hugo/cache/filecache"
+	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/media"
 	"github.com/gohugoio/hugo/output"
 	"github.com/gohugoio/hugo/resources"
 	"github.com/spf13/afero"
-	"github.com/spf13/viper"
 )
 
 func NewTestResourceSpec() (*resources.Spec, error) {
-	cfg := viper.New()
+	cfg := config.New()
 	cfg.Set("baseURL", "https://example.org")
 	cfg.Set("publishDir", "public")
 

resources/resource_transformers/js/options.go

@@ -20,9 +20,9 @@ import (
 	"path/filepath"
 	"strings"
 
-	"github.com/spf13/afero"
-
+	"github.com/gohugoio/hugo/common/maps"
 	"github.com/pkg/errors"
+	"github.com/spf13/afero"
 
 	"github.com/evanw/esbuild/pkg/api"
 
@@ -30,7 +30,6 @@ import (
 	"github.com/gohugoio/hugo/hugofs"
 	"github.com/gohugoio/hugo/media"
 	"github.com/mitchellh/mapstructure"
-	"github.com/spf13/cast"
 )
 
 const (
@@ -348,7 +347,7 @@ func toBuildOptions(opts Options) (buildOptions api.BuildOptions, err error) {
 
 	var defines map[string]string
 	if opts.Defines != nil {
-		defines = cast.ToStringMapString(opts.Defines)
+		defines = maps.ToStringMapString(opts.Defines)
 	}
 
 	// By default we only need to specify outDir and no outFile

resources/testhelpers_test.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/langs"
 	"github.com/gohugoio/hugo/modules"
 
@@ -22,7 +23,6 @@ import (
 	"github.com/gohugoio/hugo/resources/page"
 	"github.com/gohugoio/hugo/resources/resource"
 	"github.com/spf13/afero"
-	"github.com/spf13/viper"
 )
 
 type specDescriptor struct {
@@ -31,8 +31,8 @@ type specDescriptor struct {
 	fs      afero.Fs
 }
 
-func createTestCfg() *viper.Viper {
-	cfg := viper.New()
+func createTestCfg() config.Provider {
+	cfg := config.New()
 	cfg.Set("resourceDir", "resources")
 	cfg.Set("contentDir", "content")
 	cfg.Set("dataDir", "data")