[ticket/11465] Use extension finder when adding extensions' acp modules

The method acp_modules::get_module_infos() needs to use the extension
finder whenever it is looking for a module's info file. While
transitioning to the new extension system, only the initial search for all
module info files was changed to the new system. Due to this it is not
possible to add an extension's acp/mcp/ucp module manually in the ACP.
This patch will always use the extension finder for the acp module's info
files and therefore properly find the needed file. Additionally, the code
has been cleaned up a little bit.

It's missing some tests but I'm currently not sure how we'd be able to achieve that. I'm pretty sure that it's not possible to add a functional test with an extension that is inside the tests folder, or is it?

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11465

PHPBB3-11465