support/scripts/pkg-stats: properly ignore CVEs in <pkg>_IGNORE_CVES

support/scripts/pkg-stats: properly ignore CVEs in <pkg>_IGNORE_CVES

It seems like throughout the series that the CVE pkg-stats support
went through, the support for ignoring CVEs in the per-package
<pkg>_IGNORE_CVES variable was forgotten.

Let's re-introduce this, which is now very simple thanks to the CVE
class, its .identifier() propertly and the .is_cve_ignored() method of
the Package class

Cc: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>