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_test.go

@@ -0,0 +1,40 @@
+// 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 (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestCompositeConfig(t *testing.T) {
+	c := qt.New(t)
+
+	c.Run("Set and get", func(c *qt.C) {
+		base, layer := New(), New()
+		cfg := NewCompositeConfig(base, layer)
+
+		layer.Set("a1", "av")
+		base.Set("b1", "bv")
+		cfg.Set("c1", "cv")
+
+		c.Assert(cfg.Get("a1"), qt.Equals, "av")
+		c.Assert(cfg.Get("b1"), qt.Equals, "bv")
+		c.Assert(cfg.Get("c1"), qt.Equals, "cv")
+		c.Assert(cfg.IsSet("c1"), qt.IsTrue)
+		c.Assert(layer.IsSet("c1"), qt.IsTrue)
+		c.Assert(base.IsSet("c1"), qt.IsFalse)
+	})
+}