Move internal packages to an internal folder

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

internal/reader/opml/opml.go

@@ -0,0 +1,73 @@
+// 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"
+	"strings"
+)
+
+// Specs: http://opml.org/spec2.opml
+type opmlDocument struct {
+	XMLName  xml.Name              `xml:"opml"`
+	Version  string                `xml:"version,attr"`
+	Header   opmlHeader            `xml:"head"`
+	Outlines opmlOutlineCollection `xml:"body>outline"`
+}
+
+func NewOPMLDocument() *opmlDocument {
+	return &opmlDocument{}
+}
+
+type opmlHeader struct {
+	Title       string `xml:"title,omitempty"`
+	DateCreated string `xml:"dateCreated,omitempty"`
+	OwnerName   string `xml:"ownerName,omitempty"`
+}
+
+type opmlOutline struct {
+	Title    string                `xml:"title,attr,omitempty"`
+	Text     string                `xml:"text,attr"`
+	FeedURL  string                `xml:"xmlUrl,attr,omitempty"`
+	SiteURL  string                `xml:"htmlUrl,attr,omitempty"`
+	Outlines opmlOutlineCollection `xml:"outline,omitempty"`
+}
+
+func (o *opmlOutline) IsSubscription() bool {
+	return strings.TrimSpace(o.FeedURL) != ""
+}
+
+func (o *opmlOutline) GetTitle() string {
+	if o.Title != "" {
+		return o.Title
+	}
+
+	if o.Text != "" {
+		return o.Text
+	}
+
+	if o.SiteURL != "" {
+		return o.SiteURL
+	}
+
+	if o.FeedURL != "" {
+		return o.FeedURL
+	}
+
+	return ""
+}
+
+func (o *opmlOutline) GetSiteURL() string {
+	if o.SiteURL != "" {
+		return o.SiteURL
+	}
+
+	return o.FeedURL
+}
+
+type opmlOutlineCollection []opmlOutline
+
+func (o opmlOutlineCollection) HasChildren() bool {
+	return len(o) > 0
+}