pkg(7): when bootstrapping first search for pkg.pkg file then pkg.txz

The package extension is going to be changed to .pkg to be among other
things resilient to the change of compression format used and reduce
the impact of all third party tool of that change.

Ensure the bootstrap knows about it

Reviewed by: manu
Differential revision: https://reviews.freebsd.org/D29232

(cherry picked from commit a2aac2f5e5642740507a3cadb98046f3dd434ce4)
(cherry picked from commit c244b1d8a38731041d0f3ff4191192a85dd8608b)