Creating an unversioned rust-cargo symbol

[Richard Sent]

Hi Guix,

There isn't an unversioned symbol for rust-cargo defined or exported by
the (gnu packages crates-io) module. I think it would be best if an
unversioned rust-cargo symbol was created that was equal to the highest
rust-cargo release. (i.e. rust-cargo, not rust-cargo-0.76)

Alternatively, since (seemingly) only one rust-cargo package is present
at a time, perhaps the version information could be removed outright and
only an unversioned symbol used.

The alternative, (specification->package), does not work in all
circumstances (primarily with -L) [1]. This would also make Scheme code
that refers to rust-cargo cleaner, either not needing updating every new
version when the symbol changes or removing potentially awkward
(specification->package) calls.

I see that most/all of the symbols in crates-io are versioned. While
from a user perspective it would be nice if there were more unversioned
symbols, rust-cargo is /probably/ the one that needs it the most since
it's so central to the ecosystem. I can also see why versioning every
symbol might help with development given Rust's release schedule and
bootstrapping chain.

This would follow the pattern of linux-libre.

Posting to guix-devel instead of bug-guix since I'm not entirely sure
this is a bug or working as intended.

[1] https://lists.gnu.org/archive/html/guix-devel/2022-12/msg00310.html

-- 
Take it easy,
Richard Sent
Making my computer weirder one commit at a time.