require加载机制
https://www.cnblogs.com/softidea/p/5242941.html
卸载已经require的模块
https://blog.youkuaiyun.com/qq_28412897/article/details/82883133
https://www.cnblogs.com/softidea/p/5242941.html
https://blog.youkuaiyun.com/qq_28412897/article/details/82883133