[ticket/11574] Fix duplicate class loading in updater

acp_modules is abusing autoloading to load files that do not contain the classes
belonging to the filename. Since the updater registers two autoloaders, it will
include both the current and the new version of the file.

By making the class actually exist, the exit condition of the autoloader will
kick in, and the crisis is averted.

PHPBB3-11574