We should move as many of the "require" dependencies from phpbb/phpbb into phpbb/phpbb-core and require phpbb/phpbb-core in phpbb/phpbb. The goal being that composer.json in phpBB can be modified by users and updated without significant conflicts.

As part of this "phpbb/phpbb" can no longer just easily replace phpbb/phpbb-core as this will skip all of its dependencies, so we need to come up with another way to handle this.