gh-142029: Return None instead of crashing on nonexsistent module name given to create_builtin()

[dr-carlos]

• Prior to 1e4e59b, Py_NewRef(Py_None) was returned in this case; so, theoretically, this should be re-introducing behaviour that was previously broken by that commit.

• Issue: Assertion failures from calling create_builtin with invalid object #142029

[dr-carlos]

On further inspection, is_core_module() is only defined under an #ifndef NDEBUG statement (since it was only used in assert() calls), so it's not valid to use in non-debug builds. Would it be acceptable to move this out of the #if?

[dr-carlos]

On further inspection, is_core_module() is only defined under an #ifndef NDEBUG statement (since it was only used in assert() calls), so it's not valid to use in non-debug builds. Would it be acceptable to move this out of the #if?

No longer relevant for the improved implementation. Thanks to @itamaro.

[dr-carlos]

Tests seem to be failing on Android/iOS because pickled __loader__ is not builtins.__loader__. This is probably because __loader__.create_module calls _imp.create_builtin(), which was modified in this PR.

Unfortunately it's not reproducible on my Linux computer (hence the Linux tests passing fine). Not sure if it's trying to compare the new __loader__ with the old __loader__ from a previous Python version?

[vstinner]

I prefer raising an exception as done in #142033 instead of returning None.