Do not use GET_FILENAME_COMPONENT + NAME_WE to get the base filename

The problem is that NAME_WE treats everything after the first dot as file extension
but we know the extension will just be .po so in case of a file named
plasma_package_org.kde.activityswitcher.po
we want
plasma_package_org.kde.activityswitcher to be the basename and not plasma_package_org

I am pretty sure GETTEXT_PROCESS_POT_FILE and GETTEXT_CREATE_TRANSLATIONS macros have a similar
problem but since we do not use them in KDE I have no way to test it in a meaningful way

Acked by Alex

CCMAIL: kde-buildsystem@kde.org