bug#40894: import crate: Use proper variable names

[Hartmut Goebel <h.goebel@crazy-compilers.com>]

"import crate -r" shall create and use "proper" variable names for the
package definitions it prints out and for the packages it reference.

If guix commits to use variable names follow the Cargo book (see
<https://lists.gnu.org/archive/html/guix-devel/2020-04/msg00394.html>),
the importer shall create respective variables and use these names. This
would easy importing packages a lot.

Dummy example:

$guix import crate -r h2@0.2.4
…
(define-public rust-h2
…
    (arguments
      `(#:cargo-inputs
        (("rust-bytes" ,rust-bytes)
         ("rust-fnv" ,rust-fnv)

Shall become

$guix import crate -r h2@0.2.4
…
(define-public rust-h2-0.2
…
        (("rust-bytes" ,rust-bytes-0.4)
         ("rust-fnv" ,rust-fnv-1)


-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |