Move out drupal_alter('system_info' from_system_rebuild_module_data to make tests faster

In my version of the system removal patch, I made _system_rebuild_module_data into something that rarely changed and so could be stored and that speed up testing greatly. Given how much testruns we do lately and how slow they got, re-adding this feature I think is critical.

The code is there, it took a great effort to get it pass, it just needs to be re-added. Surely it was scope creep to have it in the {system} issue but I have no doubts sun only removed such a speedup to add it later.