rust: Support `make dist` -> cargo vendor

rust: Support `make dist` -> cargo vendor

What we do here basically is set things up in a `dist-hook` so that our Rust
sources are vendored at `dist` time. This gives us a single tarball still, and
ideally should be transparent to downstream builders, as long as they have the
`cargo/rust` toolchain.

Closes: #669
Approved by: jlebon