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
