On a warm cache, "guix lint -c cve vorbis-tools" goes down
from 6.5s to 2.4s.

* guix/cve.scm (cpe->package-name): Change to return two values instead
of a pair.
(cpe->product-alist): New procedure.
(%parse-vulnerability-feed): Use it instead of 'filter-map'.
(fetch-vulnerabilities): Bump sexp format version to 1.
(vulnerabilities->lookup-proc): Adjust accordingly.  When #:version is
omitted, return a list of vulnerabilities instead of a list of
version/vulnerability pairs.
* tests/cve.scm (%expected-vulnerabilities)
("vulnerabilities->lookup-proc): Adjust accordingly.