Move internal packages to an internal folder

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

internal/http/route/route.go

@@ -0,0 +1,36 @@
+// SPDX-FileCopyrightText: Copyright The Miniflux Authors. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package route // import "miniflux.app/v2/internal/http/route"
+
+import (
+	"strconv"
+
+	"github.com/gorilla/mux"
+	"miniflux.app/v2/internal/logger"
+)
+
+// Path returns the defined route based on given arguments.
+func Path(router *mux.Router, name string, args ...interface{}) string {
+	route := router.Get(name)
+	if route == nil {
+		logger.Fatal("[Route] Route not found: %s", name)
+	}
+
+	var pairs []string
+	for _, arg := range args {
+		switch param := arg.(type) {
+		case string:
+			pairs = append(pairs, param)
+		case int64:
+			pairs = append(pairs, strconv.FormatInt(param, 10))
+		}
+	}
+
+	result, err := route.URLPath(pairs...)
+	if err != nil {
+		logger.Fatal("[Route] %v", err)
+	}
+
+	return result.String()
+}