In `-sMODULARIZE` mode the `moduleArg` should not be modified in place

Right now you can observe the new module object propertires before the module promise resolves, breaking the encapsulation.

I tried to avoid this in the past but its make very hard due the way fact that we `return moduleArg` at the end of the generated code and returning anything else is tricky when building with `--closure=1`.