Read deps from Puppetfiles in git repos

It is rather annoying to have to release a module just so you can have it depend on other modules.  We've got an internal tree of modules that we can easily reference between, and it would be nice to be able to use Puppetfiles inside those module trees to do the dereferencing.  Well, now you can.

I avoided trying to put git support directly into ModuleTool, because that way lies madness (and CLAs).  I'm also rather conflicted about the use of external git repos in the test suite, but I decided to stick with existing practice rather than try to do anything clever like putting my test repos inside.