pkg: prefer .pkg extension

.pkg is the default extension as of commit c244b1d8a387, falling back to
.txz if not found.

PR: 281924
Reviewed by: bapt
Fixes: a2aac2f5e564 ("pkg(7): when bootstrapping first search for pkg.bsd file then pkg.txz")
Fixes: c244b1d8a387 ("pkg: settle the uniq extension to .pkg instead of .bsd")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46977