Replace all usage of CopyOnWriteFs with OverlayFs

Fixes #9761

hugofs/language_merge.go

@@ -0,0 +1,39 @@
+// Copyright 2022 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 hugofs
+
+import (
+	"os"
+)
+
+// LanguageDirsMerger implements the overlayfs.DirsMerger func, which is used
+// to merge two directories.
+var LanguageDirsMerger = func(lofi, bofi []os.FileInfo) []os.FileInfo {
+	for _, fi1 := range bofi {
+		fim1 := fi1.(FileMetaInfo)
+		var found bool
+		for _, fi2 := range lofi {
+			fim2 := fi2.(FileMetaInfo)
+			if fi1.Name() == fi2.Name() && fim1.Meta().Lang == fim2.Meta().Lang {
+				found = true
+				break
+			}
+		}
+		if !found {
+			lofi = append(lofi, fi1)
+		}
+	}
+
+	return lofi
+}