Add checks for the header in `packages.validate`

The method validate checks that the .po file is syntactically correct using msgfmt.

Add other checks such as the header is correct. So:

# Catalan translation of PACKAGE_NAME debconf templates.
# Copyright © 2010 Software in the Public Interest, Inc.
# This file is distributed under the same license as PACKAGE_NAME's packaging.
# THE LAST TRANSLATION IS AS EXPECTED MATCHING `Last-Translator: poc senderi <pocsenderi@protonmail.com>\n`

And other things such as:

"Language: EXPECTED\n"
"Language-Team: EXPECTED <EXPECTED>\n"