AutoloadGenerator.php: Update 'AutoloadClasses' in extension.json
AutoloadGenerator.php until now only updated autoload.php. As we shift
to extension registration, it would be cleaner if AutoloadGenerator.php
directly updates 'AutoloadClasses' in extension.json.
Bug: T88194
Change-Id: Idd601d7897634fae8c69f4d5338d9fc8f8b8f89b