Use system_list() for drupal_get_filename()

#623992: Reduce {system} database hits just got committed. We could consider using that for drupal_get_filename() too, since all the information required is already loaded, it'd just need an extra array in system_list() then using that instead of the direct query. Also it'd be worth working out if we actually need all the file_exists() checks there - we might for when modules get deleted from the modules directory without being disabled, but still.