Support gitlab in "go get" import path

With the help of this patch, you can use "go get" command to download
and install remote gitlab Go project, for example:

  go get gitlab.abc.com/<user>/<project>

It'll use git(ssh) protocol, instead of http or https, to access the
given project.

By setting your local ssh key properly, you can download any gitlab
Go project, without entering password verbosely, no matter it is
public or private.