Eli Schwartz authored May 22, 2019 and Allan McRae committed May 28, 2019

These variables must begin with .src.tar / .pkg.tar respectively, so
fail early if those expectations are not matched. This prevents makepkg
from creating e.g. package files literally named "./pacman-5.1.3-1-x86_64"
which are actually uncompressed tarballs.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>
Signed-off-by: Allan McRae <allan@archlinux.org>