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

cache/filecache/filecache_config.go

@@ -19,6 +19,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/maps"
+
 	"github.com/gohugoio/hugo/config"
 
 	"github.com/gohugoio/hugo/helpers"
@@ -123,6 +125,9 @@ func DecodeConfig(fs afero.Fs, cfg config.Provider) (Configs, error) {
 	_, isOsFs := fs.(*afero.OsFs)
 
 	for k, v := range m {
+		if _, ok := v.(maps.Params); !ok {
+			continue
+		}
 		cc := defaultCacheConfig
 
 		dc := &mapstructure.DecoderConfig{
@@ -137,7 +142,7 @@ func DecodeConfig(fs afero.Fs, cfg config.Provider) (Configs, error) {
 		}
 
 		if err := decoder.Decode(v); err != nil {
-			return nil, err
+			return nil, errors.Wrap(err, "failed to decode filecache config")
 		}
 
 		if cc.Dir == "" {

cache/filecache/filecache_config_test.go

@@ -25,7 +25,6 @@ import (
 	"github.com/gohugoio/hugo/config"
 
 	qt "github.com/frankban/quicktest"
-	"github.com/spf13/viper"
 )
 
 func TestDecodeConfig(t *testing.T) {
@@ -178,8 +177,8 @@ dir = "/"
 	c.Assert(err, qt.Not(qt.IsNil))
 }
 
-func newTestConfig() *viper.Viper {
-	cfg := viper.New()
+func newTestConfig() config.Provider {
+	cfg := config.New()
 	cfg.Set("workingDir", filepath.FromSlash("/my/cool/hugoproject"))
 	cfg.Set("contentDir", "content")
 	cfg.Set("dataDir", "data")