解决golang.org/x包无法下载的问题

git clone https://github.com/golang/net.git  $GOPATH/src/github.com/golang/net
git clone https://github.com/golang/sys.git  $GOPATH/src/github.com/golang/sys
git clone https://github.com/golang/tools.git  $GOPATH/src/github.com/golang/tools

2. 将net、sys、tools三个文件夹放到$GOPATH/src/golang.org/x目录下。