multi module, use addModuleResource to create api do not work, error message is "class xxxxController does not exist"

my project's structure like that: public/ public/index.php ishgo/index/ ishgo/index/Module.php ishgo/index/controllers/ ishgo/index/controllers/UserController.php

I want to know why the users api do not work, always show the error message "class UserController does not exist"... please help!

public/index.php

<?php

use \Phalcon\Mvc\Router,
    \Phalcon\Mvc\Application,
    \Phalcon\DI\FactoryDefault,
    \Phalcon\Db\Adapter\Pdo\Mysql;

try {

    //define app path
    define('ROOT_PATH', dirname(__DIR__));
    define('APP_PATH', ROOT_PATH . '/ishgo');

    $di = new FactoryDefault();
    $di->set('router', function () {
        $router = new \Phalcon\Mvc\Router\Annotations();
        $router->setDefaultModule('index');
        $router->addModuleResource('index', 'User', '/api/users');

        return $router;
    });

    $di->set('db', function() {
        return new Mysql(array(
            'host'     => 'localhost',
            'username' => 'root',
            'password' => '',
            'dbname'   => 'ishgo'
        ));
    });

    //Create an application
    $application = new Application($di);

    // Register the installed modules
    $application->registerModules(
        array(
            'index' => array(
                'className' => 'Ishgo\Index\Module',
                'path'      => APP_PATH . '/index/Module.php',
            ),
            'admin' => array(
                'className' => 'Ishgo\Admin\Module',
                'path'      => APP_PATH . '/admin/Module.php',
            ),
            'work'  => array(
                'className' => 'Ishgo\Work\Module',
                'path'      => APP_PATH . '/work/Module.php',
            )
        )
    );

    //add common functions
    include APP_PATH . '/functions.php';
    include ROOT_PATH . '/vendor/autoload.php';
    session_start();
    //Handle the request
    echo $application->handle()->getContent();

} catch(\Exception $e) {
    echo $e->getMessage();
}

ishgo/index/Module.php <?php

namespace Ishgo\Index;

use Phalcon\Loader,
    Phalcon\Mvc\Dispatcher,
    Phalcon\Mvc\View,
    Phalcon\Mvc\ModuleDefinitionInterface;

class Module implements ModuleDefinitionInterface
{

    /**
     * Register a specific autoloader for the module
     */
    public function registerAutoloaders()
    {

        $loader = new Loader();

        $loader->registerNamespaces(
            array(
                'Ishgo\Index\Controllers' => APP_PATH . '/index/controllers/',
                'Ishgo\Index\Models'      => APP_PATH . '/index/models/',
            )
        );

        $loader->register();
    }

    /**
     * Register specific services for the module
     */
    public function registerServices($di)
    {

        //Registering a dispatcher
        $di->set('dispatcher', function() {
            $dispatcher = new Dispatcher();
            $dispatcher->setDefaultNamespace('Ishgo\Index\Controllers');
            return $dispatcher;
        });

        //Registering the view component
        $di->set('view', function() {
            $view = new View();
            $view->setViewsDir(APP_PATH . '/index/views/');
            return $view;
        });
    }

}

ishgo/index/controllers/UserController.php

<?php
namespace Ishgo\Index\Controllers;

/**
 * @RoutePrefix("/api/users")
 */
class UserController extends \Phalcon\Mvc\Controller
{
    /**
     * @Get("/{id:\d*}")
     */
    public function indexAction($id=0)
    {
        if (empty($id)) {
            echo 'all users';
        } else {
            echo 'user by id:' . $id;
        }

    }

    /**
     * @Post("/{id:\d+}")
     */
    public function saveAction($id)
    {
        echo 'update user by id:' . $id;
    }

    /**
     * @Put("/")
     */
    public function addAction()
    {
        echo 'add user';
    }

    /**
     * @Delete("/{id:\d+}")
     */
    public function deleteAction($id)
    {
        echo 'delete user by id:' . $id;
    }

    /**
     * @Post("/login")
     */
    public function loginAction()
    {
        echo 'user login';
    }

}