  These examples/tests are for when the SWIG generated wrapper module is split
between two packages.  Specifically the pure python part is part of a package
and the C/C++ part is not in any package at all.  Historically SWIG has
supported this sort of thing.
From SWIG 4.0.0 onwards, split modules are not supported by default.
The %module directive needs to be customised with the moduleimport attribute
in order to import a global C/C++ module.

vanilla        # "plane Jane" module both halves in pkg1
vanilla_split  # python 1/2 in pkg1 C 1/2 in global namespace