rustpkg: Handle local git repositories

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.
auto merge of #7681 : catamorphism/rust/rustpkg-local-repos, r=catamorphism

r? @graydon 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.