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

[sbc100]

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.