Setting ADIOS2_USE_Python=ON doesn't error when mpi4py is not found

[KyleFromKitware]

Describe the bug
If ADIOS2_USE_Python is ON, and ADIOS2_USE_MPI is ON, but mpi4py is not present, configuration does not error like it should.

To Reproduce

1. Make sure Python development files are installed but mpi4py is not
2. Configure with ADIOS2_USE_Python=ON and ADIOS2_USE_MPI=ON
3. Observe that no error occurs

Expected behavior
An error should occur, since the required mpi4py was not found

Additional context
This appears to be due to how find_package(PythonModule) works. It handles variables for PythonModule_<module> rather than PythonModule. So when REQUIRED is passed, PythonModule_FIND_REQUIRED is set to true, but find_package_handle_standard_args() reads PythonModule_mpi4py_FIND_REQUIRED, which is not set, and so the error is not triggered.

I believe that the solution is to either change how find_package(PythonModule) works, or write a dedicated find module for mpi4py.

[KyleFromKitware]

Cc: @chuckatkins