BuildStream's vendoring doesn't deal with arbitrarily referenced
repositories; because of "cargo vendor" it depends on crates that come
from crates.io.  Since the subclass branch depends on glib-rs master,
we'll revert this whole branch temporarily until glib-rs makes a
release.

(I can't force-push to master a clean revert, hence this revert-changes commit)

Reverts merge a8f9996e