TMI: rustpkg can find files

Today's work was on bringing the reality of rustpkg up to the point described in my doc pull request, at least up to the point of the directory and file structure of packages and having rustpkg look for package source files in a src directory inside a workspace. Also, now it uses a one build directory per workspace for putting object files and other temporary files in, instead of one per package. A workspace is just a container for packages; right now, always a directory on your local filesystem, but in the future, also on github or some other remote server containing version control repositories.

Now most of the test scenarios that I checked in build, and I've crossed off one more task on the schedule. Yay crossing things off. This pull request contains the work I'm talking about.