pkg: prefer .pkg extension

[emaste]

Summary

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

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")