Move internal packages to an internal folder

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

internal/reader/opml/parser.go

@@ -0,0 +1,44 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package opml // import "miniflux.app/v2/internal/reader/opml"
+
+import (
+	"encoding/xml"
+	"io"
+
+	"miniflux.app/v2/internal/errors"
+	"miniflux.app/v2/internal/reader/encoding"
+)
+
+// Parse reads an OPML file and returns a SubcriptionList.
+func Parse(data io.Reader) (SubcriptionList, *errors.LocalizedError) {
+	opmlDocument := NewOPMLDocument()
+	decoder := xml.NewDecoder(data)
+	decoder.Entity = xml.HTMLEntity
+	decoder.Strict = false
+	decoder.CharsetReader = encoding.CharsetReader
+
+	err := decoder.Decode(opmlDocument)
+	if err != nil {
+		return nil, errors.NewLocalizedError("Unable to parse OPML file: %q", err)
+	}
+
+	return getSubscriptionsFromOutlines(opmlDocument.Outlines, ""), nil
+}
+
+func getSubscriptionsFromOutlines(outlines opmlOutlineCollection, category string) (subscriptions SubcriptionList) {
+	for _, outline := range outlines {
+		if outline.IsSubscription() {
+			subscriptions = append(subscriptions, &Subcription{
+				Title:        outline.GetTitle(),
+				FeedURL:      outline.FeedURL,
+				SiteURL:      outline.GetSiteURL(),
+				CategoryName: category,
+			})
+		} else if outline.Outlines.HasChildren() {
+			subscriptions = append(subscriptions, getSubscriptionsFromOutlines(outline.Outlines, outline.Text)...)
+		}
+	}
+	return subscriptions
+}