TMI: rustpkg and searching the current directory

Today, implemented #6405; in general, rustpkg commands should do something without an explicit package ID given on the command line. If you just say rustpkg build (or any other command), rustpkg should default to treating the current directory as a subdirectory of the source directory in a workspace, and building the package in the current directory. I submitted a pull request to fix that.

I also went through the 0.7 bugs and added test cases for the ones that had been mysteriously-fixed, but my branch where I'm adding the test cases fails in tests that don't have anything to do with anything I changed (and anyway, I only changed other test cases) so... for now, I'm stumped on that. Is anyone else seeing the rusttest unit tests for the rust command-line tool ICE?)

In the meantime, bors is slow and even my pull requests from yesterday haven't built yet :-(