Cargo plugin can't handle git sources

looks like it currently is only able to fetch/vendor crates from crates.io.

[dependencies]
libhandy = { git = "https://gitlab.gnome.org/World/Rust/libhandy-rs.git" }

1816        [00:00:14] FAILURE core/gnome-tour.bst: <pluginbase._internalspace._sp332110df150de94e42a03271327a4631.cargo.Crate object at 0x7fb5751c6950>: Error mirroring https://static.crates.io/crates/libhandy/libhandy-1.0.0.crate: HTTP Error 403: Forbidden