阿里云镜像：
全局配置：
composer config -g repo.packagist composer https://mirrors.aliyun.com/composer/
取消配置：
composer config -g --unset repos.packagist
项目配置（仅针对当前工程）：
composer config repo.packagist composer https://mirrors.aliyun.com/composer/
取消配置：
composer config -g --unset repos.packagist