Mk/Uses/cargo.mk: Fix crate name and version split

cargo.mk is a little too eager in splitting a crate's name and version
in two when the version contains a dash (e.g. csv-1.0.0-beta.4) and
the wrong URL ends up being used in MASTER_SITES e.g.

.../csv-1.0.0/beta.4/download/...

instead of the correct one

.../csv/1.0.0-beta.4/download/...

Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D12628