Codeigniter 3 HMVC：无法加载会话库

Here is the folder structure in modules library

1. users->controllers->Users.php

   users->config->autoload.php

2. welcome->controllers->Welcome.php

autoload.php

$autoload = array(
    'helper'    => array('url', 'form'),
    'libraries' => array('session')
);

Welcome.php (located in modules/welcome/Welcome.php)

class Welcome extends MX_Controller {

   public function index()
    {
       //load module
       $this->load->module(array('users'));
    }
}

I will get the following error :

Unable to locate the specified class: Session.php

Note: url and form libraries are loaded correctly

In config/autoload.php file put below line

$autoload['libraries'] = array('database','form_validation','session');   

Its work fine for me.