Mk/Uses/cargo.mk: Use canonical crate file extension going forward

Crates should have the file extension .crate not .tar.gz. Cargo
saves them with that extension and it is also what their original
name was according to file(1):

$ file rust/crates/*
rust/crates/better-panic-0.2.0.tar.gz:gzip compressed data, was "better-panic-0.2.0.crate"
rust/crates/binary-space-partition-0.1.2.tar.gz:gzip compressed data, was "binary-space-partition-0.1.2.crate"
...

The only reason we used .tar.gz is that it was that way in OpenBSD's
devel/cargo/cargo.port.mk module. At the moment they are all .tar.gz
but they might have a different format in the future.

Doing a delayed rollout of this over a change-everything-once
approach to

• not immeditately invalidate all uncommitted port changes or external ports
• not make sweeping changes twice in main and 2021Q2

at the cost of having a transition period where mirrors might have
to carry both the .tar.gz and .crate tarballs (all crates combined
are ~600 MiB).

Reviewed by: jbeich
MFH: 2021Q2 (to ease future cherry picks)
Differential Revision: https://reviews.freebsd.org/D29760