解决方法：
进入到上面说的golang.org/x目录下，执行命令下载sys。
注意：提前安装好Git，我本地的GOPATH目录为：D:\go\GoProjects
(1) 先配置一下Git，不然会报错：OpenSSL SSL_read: Connection was reset, errno 10054
git config --global http.sslVerify "false"
(2) 下载官方扩展包：
cd D:\go\GoProjects\src\golang.org\x\ git clone https://github.com/golang/tools.git git clone https://github.com/golang/net.git git clone https://github.com/golang/sys.git git clone https://github.com/golang/term.git git clone https://github.com/golang/crypto.git git clone https://github.com/golang/oauth2.git git clone https://github.com/golang/blog.git git clone https://github.com/golang/text.git git clone https://github.com/golang/review.git git clone https://github.com/golang/time.git git clone https://github.com/golang/vgo.git git clone https://github.com/golang/image.git git clone https://github.com/golang/sync.git