MDL-42400 modlib: include module file when it's used

- changed the place where module file is included to make sure that add_moduleinfo() can be called independently from create_module()
- removed permission check from add_moduleinfo() because this function is not supposed to check permissions. Besides it's only half-check. See can_add_moduleinfo()
- added phpdocs to some functions to indicate that they can throw an exception
MDL-42400 changes to generator (still nees polishing)