Ticket/12783 Move require:phpbb/phpbb to extra:phpbb/phpbb in Extensions composer.json files

Alternative2: #2683

https://tracker.phpbb.com/browse/PHPBB3-12783
@VSEphpbb It's always better to have a vendor in there, even if it's not explicitly required. phpbb/phpbb in itself is little redundant but is necessary (and we aren't the only ones `composer/composer`, `symfony/symfony`).