Porting part of Composer's filesystem to get around rename issue

Hey Raul!

I'm interested in "fixing" the rename problem that's causing issues on symfony/symfony-installer#46. Obviously, this is not Distill's issue, it should actually be fixed in Symfony. But, my worry is that it might take longer to get into Symfony, especially since the Composer logic we'll need to port currently relies on the Process component, and I think Symfony will want to remove that dependency.

So, I'm proposing that we temporarily port the Composer logic directly into Distill. I've done that here. The downside is that *we* now have a dependency on the Process component. My hope is that you'll think that's ok, since it fixes this bug (and is temporary).

Thanks!
Never noticed before, but why not simply use `~2.4` here?