rustpkg: Start teaching rustpkg to infer package IDs 

r? @graydon This is preliminary work on bringing rustpkg up to conformance with #5679
and related issues.

This change makes rustpkg infer a package ID from its containing directory,
instead of requiring name and vers attributes in the code. Many aspects of it
are incomplete; I've only tested one package (see README.txt) and one command,
"build". So far it only works for local packages.

I also removed code for several of the package commands other than "build",
replacing them with stubs that fail, since they used package IDs in ways that
didn't jibe well with the new scheme. I will re-implement the commands one
at a time.