TMI: Tag!

Submitted a pull request in the waning hours of the day, which wraps up support for tags. rustpkg now knows about tags that aren't versions. I refactored some of the git-fetching code while I was at it, and changed it so that the build output for foo goes in build/foo and not directly in build (this is more consistent, since rustpkg was already doing that for packages with compound names).

I worked more on extended extern mod names, but didn't finish that. Hacking on metadata::{filesearch, loader}: always a good time.

I also found this delightful bug to do with calling run process functions and specifying an environment, though I'm still not convinced it's not due to something weird happening on my machine. So if someone wanted to try reproducing it, that would be awesome.