perl: use <pkg>_EXTRA_DOWNLOADS

Instead of manually using the DOWNLOAD macro (which should remain an
internal macro), this commit converts the Perl package to use
<pkg>_EXTRA_DOWNLOADS, now that it has been extended to allow full
URLs.

[Thomas: as suggested by Yann, keep comment explaining how we handle
perl-cross.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Romain Naour <romain.naour@openwide.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>