Move internal packages to an internal folder

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

internal/reader/opml/serializer.go

@@ -0,0 +1,71 @@
+// 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 (
+	"bufio"
+	"bytes"
+	"encoding/xml"
+	"sort"
+	"time"
+
+	"miniflux.app/v2/internal/logger"
+)
+
+// Serialize returns a SubcriptionList in OPML format.
+func Serialize(subscriptions SubcriptionList) string {
+	var b bytes.Buffer
+	writer := bufio.NewWriter(&b)
+	writer.WriteString(xml.Header)
+
+	opmlDocument := convertSubscriptionsToOPML(subscriptions)
+	encoder := xml.NewEncoder(writer)
+	encoder.Indent("", "    ")
+	if err := encoder.Encode(opmlDocument); err != nil {
+		logger.Error("[OPML:Serialize] %v", err)
+		return ""
+	}
+
+	return b.String()
+}
+
+func convertSubscriptionsToOPML(subscriptions SubcriptionList) *opmlDocument {
+	opmlDocument := NewOPMLDocument()
+	opmlDocument.Version = "2.0"
+	opmlDocument.Header.Title = "Miniflux"
+	opmlDocument.Header.DateCreated = time.Now().Format("Mon, 02 Jan 2006 15:04:05 MST")
+
+	groupedSubs := groupSubscriptionsByFeed(subscriptions)
+	var categories []string
+	for k := range groupedSubs {
+		categories = append(categories, k)
+	}
+	sort.Strings(categories)
+
+	for _, categoryName := range categories {
+		category := opmlOutline{Text: categoryName}
+		for _, subscription := range groupedSubs[categoryName] {
+			category.Outlines = append(category.Outlines, opmlOutline{
+				Title:   subscription.Title,
+				Text:    subscription.Title,
+				FeedURL: subscription.FeedURL,
+				SiteURL: subscription.SiteURL,
+			})
+		}
+
+		opmlDocument.Outlines = append(opmlDocument.Outlines, category)
+	}
+
+	return opmlDocument
+}
+
+func groupSubscriptionsByFeed(subscriptions SubcriptionList) map[string]SubcriptionList {
+	groups := make(map[string]SubcriptionList)
+
+	for _, subscription := range subscriptions {
+		groups[subscription.CategoryName] = append(groups[subscription.CategoryName], subscription)
+	}
+
+	return groups
+}