Move internal packages to an internal folder

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

internal/reader/opml/subscription.go

@@ -0,0 +1,21 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package opml // import "miniflux.app/v2/internal/reader/opml"
+
+// Subcription represents a feed that will be imported or exported.
+type Subcription struct {
+	Title        string
+	SiteURL      string
+	FeedURL      string
+	CategoryName string
+}
+
+// Equals compare two subscriptions.
+func (s Subcription) Equals(subscription *Subcription) bool {
+	return s.Title == subscription.Title && s.SiteURL == subscription.SiteURL &&
+		s.FeedURL == subscription.FeedURL && s.CategoryName == subscription.CategoryName
+}
+
+// SubcriptionList is a list of subscriptions.
+type SubcriptionList []*Subcription