Only include .po files that has a companion .aliases file

Disabled translations have a .disabled file instead of
an .aliases file.  Without this patch I couldn't make
subsurface after a tx pull (when testing translations).

This patch makes the .aliases file an explicit requirement
for a valid translation.

Signed-off-by: Henrik Brautaset Aronsen <subsurface@henrik.synth.no>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>