Improve pip package build (#5965)

Summary:
Addressing comments in https://github.com/pytorch/executorch/issues/5961.

* Separate out `executorch-wheel-config.cmake` from `executorch-config.cmake`.
* Hardcode the envrionment variable `SETUPTOOLS_EXT_SUFFIX` in `setup.py`.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5965

Reviewed By: dbort

Differential Revision: D64017947

Pulled By: larryliu0820

fbshipit-source-id: 0bdff5e2d2ec5873540d1b701595c7a316e84e80