gh-153236: Propagate lazy submodule import errors

[pablogsal]

I have chosen to make lazy submodule resolution explicit instead of classifying the exception after a failed import. A real module can raise ModuleNotFoundError with its own name while executing, so the exception doesn reliably tell us whether the finder failed or the loader failed.

I think the superior way here is to do this in importlib's find/load boundary. No spec means the lazy submodule is absent and attribute lookup can continue. Once a spec exists, the submodule is real, so any later exception propagates normally.

• Issue: Lazy submodule imports hide exceptions raised while loading the submodule #153236

[pablogsal]

CC @brittanyrey