[ticket/12775] Replace functions_container with a container_builder class.

https://tracker.phpbb.com/browse/PHPBB3-12775
https://tracker.phpbb.com/browse/PHPBB3-12420
https://tracker.phpbb.com/browse/PHPBB3-12774
(Kind of required for https://tracker.phpbb.com/browse/PHPBB3-12685)

PHPBB3-12775
Please move that down to where require($phpbb_root_path . 'config.' . $phpEx); was . People refactoring this should not have to think about all the possible hidden data flows.