packages: Move xz to the minimal set

pkg(8), via its daily periodic script, requires xz. We don't have
a way to encode dependencies from ports on base packages right now,
so instead move xz to the minimal set so it's always installed.

This isn't an ideal solution, but it's justified in this case since
pkg(8) is always installed, so having its dependencies always
installed is acceptable.

(Following discusson on the diff, new versions of pkg have now been
changed to use zstd instead of xz, but we still think xz is useful
enough to keep in minimal.)

MFC after: 1 week (stable/15 only)
Reviewed by: des, bapt, emaste
Differential Revision: https://reviews.freebsd.org/D55630
Sponsored by: https://www.patreon.com/bsdivy

(cherry picked from commit 43773002fa63262d531ce3ee5cf5d3fe872a3388)