Mk/Uses/cargo.mk: Push lib dependencies back down into ports

Some crates are optional via Cargo features or are only used during
tests, however the framework has no way to discriminate between
test or run dependencies using just CARGO_CRATES leading to more
run dependencies than necessary for some packages. With more ported
Rust applications it's time to let individual ports make that
decision now.

The environmental setup to use dependencies from ports instead of
bundled ones and implied build dependencies (cmake, gmake, pkgconf)
are left in place for now.

Assign cargo.mk to rust@ while here.