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/compositeConfig.go

@@ -0,0 +1,113 @@
+// Copyright 2021 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package config
+
+import (
+	"github.com/gohugoio/hugo/common/maps"
+)
+
+// NewCompositeConfig creates a new composite Provider with a read-only base
+// and a writeable layer.
+func NewCompositeConfig(base, layer Provider) Provider {
+	return &compositeConfig{
+		base:  base,
+		layer: layer,
+	}
+}
+
+// compositeConfig contains a read only config base with
+// a possibly writeable config layer on top.
+type compositeConfig struct {
+	base  Provider
+	layer Provider
+}
+
+func (c *compositeConfig) GetBool(key string) bool {
+	if c.layer.IsSet(key) {
+		return c.layer.GetBool(key)
+	}
+	return c.base.GetBool(key)
+}
+
+func (c *compositeConfig) GetInt(key string) int {
+	if c.layer.IsSet(key) {
+		return c.layer.GetInt(key)
+	}
+	return c.base.GetInt(key)
+}
+
+func (c *compositeConfig) Merge(key string, value interface{}) {
+	c.layer.Merge(key, value)
+}
+
+func (c *compositeConfig) GetParams(key string) maps.Params {
+	if c.layer.IsSet(key) {
+		return c.layer.GetParams(key)
+	}
+	return c.base.GetParams(key)
+}
+
+func (c *compositeConfig) GetStringMap(key string) map[string]interface{} {
+	if c.layer.IsSet(key) {
+		return c.layer.GetStringMap(key)
+	}
+	return c.base.GetStringMap(key)
+}
+
+func (c *compositeConfig) GetStringMapString(key string) map[string]string {
+	if c.layer.IsSet(key) {
+		return c.layer.GetStringMapString(key)
+	}
+	return c.base.GetStringMapString(key)
+}
+
+func (c *compositeConfig) GetStringSlice(key string) []string {
+	if c.layer.IsSet(key) {
+		return c.layer.GetStringSlice(key)
+	}
+	return c.base.GetStringSlice(key)
+}
+
+func (c *compositeConfig) Get(key string) interface{} {
+	if c.layer.IsSet(key) {
+		return c.layer.Get(key)
+	}
+	return c.base.Get(key)
+}
+
+func (c *compositeConfig) IsSet(key string) bool {
+	if c.layer.IsSet(key) {
+		return true
+	}
+	return c.base.IsSet(key)
+}
+
+func (c *compositeConfig) GetString(key string) string {
+	if c.layer.IsSet(key) {
+		return c.layer.GetString(key)
+	}
+	return c.base.GetString(key)
+}
+
+func (c *compositeConfig) Set(key string, value interface{}) {
+	c.layer.Set(key, value)
+}
+
+func (c *compositeConfig) WalkParams(walkFn func(params ...KeyParams) bool) {
+	panic("not supported")
+}
+
+func (c *compositeConfig) SetDefaultMergeStrategy() {
+	panic("not supported")
+}