release: Rework VM package selection using sets

Filtering the list of packages broke with the introduction of package
sets, since we excluded FreeBSD-src.* but still included
FreeBSD-set-src, which transitively included the FreeBSD-src and
FreeBSD-src-sys packages. This could be fixed by excluding package
sets, but that would be fragile, potentially breaking upgrades if
new packages are introduced.

Instead, start with an explicit set of package sets:
base, base-dbg
lib32, lib32-dbg
kernels, kernels-dbg
tests
and filter the package sets; the EC2 "small" and "builder" AMIs
filter out everything except "base" and "kernels".

Note that using FreeBSD-set-kernels may pose a problem in the future
if we start shipping packages for multiple differently-configured
kernels. That will be addressed if and when that problem arises.

Reviewed by: ivy
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D52922

(cherry picked from commit 2d184465e8bb77d4620f509b04f19ea22656f28e)