I think that the groups install/upgrade code should be moved into main because the upgrade code for groups should not be executed after the main upgrade - it should be executed incrementally together with the rest of main upgrades or else it could be a major problem if future if somebody tries to upgrade from 1.7 to 2.x; the reason is that some core code (enrollments, meta courses, modules, etc.) might indirectly rely on group tables that would not be present during pgrade.