Move internal packages to an internal folder

For reference: https://go.dev/doc/go1.4#internalpackages

internal/reader/encoding/encoding.go

@@ -0,0 +1,37 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package encoding // import "miniflux.app/v2/internal/reader/encoding"
+
+import (
+	"bytes"
+	"io"
+	"unicode/utf8"
+
+	"golang.org/x/net/html/charset"
+)
+
+// CharsetReader is used when the XML encoding is specified for the input document.
+//
+// The document is converted in UTF-8 only if a different encoding is specified
+// and the document is not already UTF-8.
+//
+// Several edge cases could exists:
+//
+// - Feeds with encoding specified only in Content-Type header and not in XML document
+// - Feeds with encoding specified in both places
+// - Feeds with encoding specified only in XML document and not in HTTP header
+// - Feeds with wrong encoding defined and already in UTF-8
+func CharsetReader(label string, input io.Reader) (io.Reader, error) {
+	buffer, _ := io.ReadAll(input)
+	r := bytes.NewReader(buffer)
+
+	// The document is already UTF-8, do not do anything (avoid double-encoding).
+	// That means the specified encoding in XML prolog is wrong.
+	if utf8.Valid(buffer) {
+		return r, nil
+	}
+
+	// Transform document to UTF-8 from the specified encoding in XML prolog.
+	return charset.NewReaderLabel(label, r)
+}