Move internal packages to an internal folder

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

internal/model/home_page.go

@@ -0,0 +1,15 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package model // import "miniflux.app/v2/internal/model"
+
+// HomePages returns the list of available home pages.
+func HomePages() map[string]string {
+	return map[string]string{
+		"unread":     "menu.unread",
+		"starred":    "menu.starred",
+		"history":    "menu.history",
+		"feeds":      "menu.feeds",
+		"categories": "menu.categories",
+	}
+}