Mk/Uses/cargo.mk: Let 'cargo-crates' generate Cargo.lock if it does not exist

This will make maintaining ports easier when upstream does not
provide a Cargo.lock on its own.

It uses the crate registry and will output the newest crate versions
available based on the restrictions in Cargo.toml. As such subsequent
runs of make cargo-crates might not generate the same output.
Upstream should still be encouraged to provide Cargo.lock if possible.

Requested by: mat
Differential Revision: https://reviews.freebsd.org/D17003