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

config/commonConfig_test.go

@@ -21,14 +21,12 @@ import (
 	"github.com/gohugoio/hugo/common/types"
 
 	qt "github.com/frankban/quicktest"
-
-	"github.com/spf13/viper"
 )
 
 func TestBuild(t *testing.T) {
 	c := qt.New(t)
 
-	v := viper.New()
+	v := New()
 	v.Set("build", map[string]interface{}{
 		"useResourceCacheWhen": "always",
 	})