Progress report while loading dependencies

[jamesmunns]

Some dependencies take a bit to load. It might be good to give some kind of feedback to the user that work is in progress.

[matklad]

I think the way to do that would be to just show Cargo's default output (we just shell out to Cargo to do the actual downloading), but that's not super trivial, b/c we don't run Cargo ourselves, we use cargo-metadata crate.