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

1 files changed, 4 insertions, 7 deletions

diff --git a/related/inverted_index.go b/related/inverted_index.go
index 2e0ea295d..329a8c998 100644
--- a/related/inverted_index.go
+++ b/related/inverted_index.go
@@ -22,6 +22,8 @@ import (
 	"strings"
 	"time"
 
+	"github.com/gohugoio/hugo/common/maps"
+
 	"github.com/gohugoio/hugo/common/types"
 	"github.com/mitchellh/mapstructure"
 )
@@ -404,16 +406,11 @@ func norm(num, min, max int) int {
 }
 
 // DecodeConfig decodes a slice of map into Config.
-func DecodeConfig(in interface{}) (Config, error) {
-	if in == nil {
+func DecodeConfig(m maps.Params) (Config, error) {
+	if m == nil {
 		return Config{}, errors.New("no related config provided")
 	}
 
-	m, ok := in.(map[string]interface{})
-	if !ok {
-		return Config{}, fmt.Errorf("expected map[string]interface {} got %T", in)
-	}
-
 	if len(m) == 0 {
 		return Config{}, errors.New("empty related config provided")
 	}