官方手册中有以下代码，注册多个模块

// 注册模块
$application->registerModules(
    array(
        'frontend' => array(
            'className' => 'Multiple\Frontend\Module',
            'path'      => '../apps/frontend/Module.php',
        ),
        'backend'  => array(
            'className' => 'Multiple\Backend\Module',
            'path'      => '../apps/backend/Module.php',
        )
    )
);

其中'className' => 'Multiple\Frontend\Module',是什么意思，按官方文件配置，总显示以下：

'Multiple\Frontend\Module' wasn't found in the dependency injection container