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

helpers/content_test.go

@@ -19,12 +19,11 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/gohugoio/hugo/config"
 	"github.com/spf13/afero"
 
 	"github.com/gohugoio/hugo/common/loggers"
 
-	"github.com/spf13/viper"
-
 	qt "github.com/frankban/quicktest"
 )
 
@@ -103,7 +102,7 @@ func TestBytesToHTML(t *testing.T) {
 }
 
 func TestNewContentSpec(t *testing.T) {
-	cfg := viper.New()
+	cfg := config.New()
 	c := qt.New(t)
 
 	cfg.Set("summaryLength", 32)