Improve check_party for 1st party module

[laggykiller]

This is the original check_party() in stubgen.py

    def check_party(self, module: str) -> Literal[0, 1, 2]:
        """
        Check source of module
        0 = From stdlib
        1 = From 3rd party package
        2 = From the package being built
        """
        if module.startswith(".") or module == self.module.__name__.split('.')[0]:
            return 2

        try:
            spec = importlib.util.find_spec(module)
        except (ModuleNotFoundError, ValueError):
            return 1

        if spec:
            if spec.origin and "site-packages" in spec.origin:
                return 1
            else:
                return 0
        else:
            return 1

See https://github.com/wjakob/nanobind/issues/420#issuecomment-2043426430

Somehow module in check_party could be my_sanitized_module.BASE instead of .my_sanitized_module or my_sanitized_module. https://github.com/wjakob/nanobind/pull/496 only prevent crash due to ValueError, but all those modules are all misclassified as 3rd party modules instead of 1st party. This PR improve check for such condition.

[wjakob]

Thanks!