[mlir] Limit Python dependency to Development.Module when possible.

After CMake 3.18, we are able to limit the scope of the search to just
Development.Module. Searching for Development will fail in situations
where the Python libraries are not available. When possible, limit to
just Development.Module. See:
https://pybind11.readthedocs.io/en/stable/compiling.html#findpython-mode

Reviewed By: stellaraccident

Differential Revision: https://reviews.llvm.org/D111585