TMI: Think globally, build locally

Submitted a pull request today! It's been a while. This one eliminates the dependency some of the rustpkg tests had on remote URLs, specifically github. Since Github tends to go down a lot, that was causing spurious test failures.

To fix it, I made rustpkg handle fetching from a local git repository, and changed the tests to test that instead. If you supply rustpkg with a package ID referring to a github repo that's outside the RUST_PATH, it will clone that into a workspace in the RUST_PATH and build from there.

I can now go home feeling a bit more accomplished.