TMI: Caching, duplicate packages, and refactoring

Since my last update, I've landed #7681, which adds support to rustpkg for local git repositories and changes the test suite to use them (getting rid of intermittent failures due to network breakage), and #7749, to implement the list and uninstall commands.

Now I'm working on a few different things at once:

• Started trying to figure out how the mysterious workcache module works, as a prerequisite for 7075 (the "make rustpkg actually not rebuild everything all the time" bug).
• With #7681 landed, tried to un-block my pending pull request for making rustpkg build work without a package argument. I'm noticing that install doesn't actually check that a package with the same package ID isn't already installed, so I have to fix that, even though it's not related.
• For #6407, I have to move some code from rustpkg into rustc so that rustc itself knows how to use the RUST_PATH. Working on that.