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.
rustpkg/api.rs provides functions intended for package scripts to call.
It will probably need more functionality added to it later, but this is
a start.
Added a test case checking that a package script can use the API.
Closes#6401
