Streamline ModuleLoader.merge_globals_lexically
Streamline ModuleLoader.merge_globals_lexically

- revert known symbols abstraction, turned out it wasn't general enough
- rename is_stub to is_HOW_stub, it doesn't take HOWs anymore
- don't do anything if there's nothing to do
- group actions together and simplify checking logic
- remove P6M from error message

rakudo/rakudo,rakudo/rakudo,MasterDuke17/rakudo,MasterDuke17/rakudo,labster/rakudo,labster/rakudo,MasterDuke17/rakudo,salortiz/rakudo,rakudo/rakudo,salortiz/rakudo,rakudo/rakudo,MasterDuke17/rakudo,labster/rakudo,MasterDuke17/rakudo,salortiz/rakudo,labster/rakudo,salortiz/rakudo,tbrowder/rakudo,rakudo/rakudo,tbrowder/rakudo,tbrowder/rakudo,tbrowder/rakudo,tbrowder/rakudo,salortiz/rakudo,tbrowder/rakudo,salortiz/rakudo,MasterDuke17/rakudo,labster/rakudo,rakudo/rakudo,labster/rakudo