Move internal packages to an internal folder

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

reader/xml/decoder.go

@@ -1,85 +0,0 @@
-// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
-// SPDX-License-Identifier: Apache-2.0
-
-package xml // import "miniflux.app/v2/reader/xml"
-
-import (
-	"bytes"
-	"encoding/xml"
-	"fmt"
-	"io"
-	"strings"
-
-	"miniflux.app/v2/reader/encoding"
-)
-
-// NewDecoder returns a XML decoder that filters illegal characters.
-func NewDecoder(data io.Reader) *xml.Decoder {
-	var decoder *xml.Decoder
-	buffer, _ := io.ReadAll(data)
-	enc := procInst("encoding", string(buffer))
-	if enc != "" && enc != "utf-8" && enc != "UTF-8" && !strings.EqualFold(enc, "utf-8") {
-		// filter invalid chars later within decoder.CharsetReader
-		decoder = xml.NewDecoder(bytes.NewReader(buffer))
-	} else {
-		// filter invalid chars now, since decoder.CharsetReader not called for utf-8 content
-		filteredBytes := bytes.Map(filterValidXMLChar, buffer)
-		decoder = xml.NewDecoder(bytes.NewReader(filteredBytes))
-	}
-
-	decoder.Entity = xml.HTMLEntity
-	decoder.Strict = false
-	decoder.CharsetReader = func(charset string, input io.Reader) (io.Reader, error) {
-		utf8Reader, err := encoding.CharsetReader(charset, input)
-		if err != nil {
-			return nil, err
-		}
-		rawData, err := io.ReadAll(utf8Reader)
-		if err != nil {
-			return nil, fmt.Errorf("Unable to read data: %q", err)
-		}
-		filteredBytes := bytes.Map(filterValidXMLChar, rawData)
-		return bytes.NewReader(filteredBytes), nil
-	}
-
-	return decoder
-}
-
-// This function is copied from encoding/xml package,
-// and is used to check if all the characters are legal.
-func filterValidXMLChar(r rune) rune {
-	if r == 0x09 ||
-		r == 0x0A ||
-		r == 0x0D ||
-		r >= 0x20 && r <= 0xD7FF ||
-		r >= 0xE000 && r <= 0xFFFD ||
-		r >= 0x10000 && r <= 0x10FFFF {
-		return r
-	}
-	return -1
-}
-
-// This function is copied from encoding/xml package,
-// procInst parses the `param="..."` or `param='...'`
-// value out of the provided string, returning "" if not found.
-func procInst(param, s string) string {
-	// TODO: this parsing is somewhat lame and not exact.
-	// It works for all actual cases, though.
-	param = param + "="
-	idx := strings.Index(s, param)
-	if idx == -1 {
-		return ""
-	}
-	v := s[idx+len(param):]
-	if v == "" {
-		return ""
-	}
-	if v[0] != '\'' && v[0] != '"' {
-		return ""
-	}
-	idx = strings.IndexRune(v[1:], rune(v[0]))
-	if idx == -1 {
-		return ""
-	}
-	return v[1 : idx+1]
-}