Fixes <https://bugs.gnu.org/38709>.
Reported by Valentin Ignatev <valentignatev@gmail.com>.
* guix/import/crate.scm (crate-recursive-import): Add optional 'version'
parameter and honor it.
* guix/scripts/import/crate.scm (guix-import-crate): Pass VERSION as 2nd
argument to 'crate-recursive-import'.
* guix/import/crate.scm (crate->guix-package): Add optional 'version'
argument and honor it.
* guix/scripts/import/crate.scm (guix-import-crate): Assume the first
argument is a spec and destructure it with
'package-name->name+version'. Pass both to 'crate->guix-package'.
* doc/guix.texi (Invoking guix import): Document it.
Co-authored-by: Ludovic Courtès <ludo@gnu.org>