composer config -g repo.packagist composer + 地址
常见镜像地址：
composer config -g repo.packagist composer https://packagist.org 国外
composer config -g repo.packagist composer https://packagist.phpcomposer.com 中国全量镜像(现在要用户名和密码才能使用了)
composer config -g repo.packagist composer https://packagist.laravel-china.org laravel社区
composer config -g repo.packagist composer https://mirrors.aliyun.com/composer/ 阿里云