Fix handling of case where spec.origin is None in get_module_members.

[copybara-service]

Fix handling of case where spec.origin is None in get_module_members.

There were a few issues here that this CL fixes:

1. We didn't explicitly import importlib.util, which meant that type-checking was not enabled for importlib.util.find_spec and its returned value.
2. The documentation (and the type info that we failed to load) state that spec.origin can be None, but we weren't checking for this.
3. We checked the existence of the file and then opened it, which is a classic filesystem race-condition.