Support for the :path keyword for git repositories

It acts the same way librarian-puppet does:

> The Git source also supports a :path => option. If we use the path option, Librarian-puppet will navigate down into the Git repository and only use the specified subdirectory. Some people have the habit of having a single repository with many modules in it. If we need a module from such a repository, we can use the :path => option here to help Librarian-puppet drill down and find the module subdirectory.