git-multipkg: add support for ssh and packages that live deep in a git repo

[cuzelac]

Add support for something like this:

$ git-multipkg -b https://github.com/ytoolshed/ range/libcrange

This is accomplished by changing a couple things:

• the repo is now cloned in to TEMPDIR/clone
• the package directory (inside the clone) is then symlinked to TEMPDIR/src
• 'pkg()' is then called on TEMPDIR/src just like always

I decided to add a 'clone' directory and use a symlink to TEMPDIR/src so as to not break convention.

A disadvantage to supporting this is that it can be slow for large repos when you only need a part of them.