pkg: Move FreeBSD-base to pkg.freebsd.org

Rather than fetching packages directly from the CDN which currently
backs pkgbase.freebsd.org, requests will go to pkg.freebsd.org mirrors
and be 302ed to the correct servers. This adds ~70 seconds to the
process of installing or upgrading a pkgbase system; it also orphans
systems with 15.0-{PRERELEASE,ALPHA*,BETA*} installed since they are
expecting to see pkgbase files signed with the pkg keys, not the new
pkgbase signing keys.

Approved by: re (cperciva)
Reviewed by: dch, philip
MFC after: immediately (for 15.0-RELEASE)
With hat: re
Requested by: clusteradm, core
Differential Revision: https://reviews.freebsd.org/D53964

(cherry picked from commit a3b0902d73901e7243103e60cf78c53cd7f566a1)
(cherry picked from commit 0c87d29a54da527b2f624cf4d2999ebeb9885f15)