Package IDs can now be of the form a/b/c#FOO, where (if a/b/c is
a git repository) FOO is any tag in the repository. Non-numeric
tags only match against package IDs with the same tag, and aren't
compared linearly like numeric versions.
While I was at it, refactored the code that calls `git clone`,
and segregated build output properly for different packages.
rustpkg can now build code from a local git repository. In the
case where the local repo is in a directory not in the RUST_PATH,
it checks out the repository into a directory in the first workspace
in the RUST_PATH.
The tests no longer try to connect to github.com, which should
solve some of the sporadic failures we've been seeing.
