fix basename handling of po files

Summary:
get_filename_component(... ... NAME_WE) gets the name without the *longest*
extension. in case of 'org.kde.plasma.kittens.po' that would leave us
with 'org' as name. Use a regular expression to grab the basename without
the short extension (e.g. 'org.kde.plasma.kittens') to prevent broken file
names.

add a test to assert that this works in the future

BUG: 379116

Reviewers: apol, ilic

Reviewed By: apol

Subscribers: #frameworks

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D5557