Maya: `**kwargs` is not overriding signatures as expected

[ColinKennedy]

There's two separate, possibly related issues

1. **kwargs is not getting expanded in the generated signature as expected all the time
2. When an explicit keyword is given such as currentNamespace in "(currentNamespace: Literal[True], **kwargs) -> str", the expanded signature lists currentNamespace twice, which is an invalid signature

Note: This ticket is a followup from 5dafefd. See the TODO note in that code for an example.