TMI: Transitive dependencies

[tim]

Today's pull request: making rustpkg find dependencies recursively. It wouldn't be much of a build system if it didn't do that, and it actually almost did without deliberate effort -- the missing piece was making the code that infers dependencies and searches for them actually search different workspaces other than the workspace of the package that has the dependencies. Other than that, it was just a matter of adding a test case, and then #8524 can close.