Find portable_lib.so in pip package during cmake build (#5961)

Summary:
* Rename `_portable_lib.cpython-3.<distribution info>.so` to `_portable_lib.so` so it can be found by CMake `find_library()`. This can be achieved by setting `SETUPTOOLS_EXT_SUFFIX`.
* Since `executorch-config.cmake` is also being used to find installed libraries such as `executorch.a`, `xnnpack_backend.a`, add a condition to tell if `executorch-config.cmake` is being used in cmake-out or site-packages.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5961

Reviewed By: metascroy

Differential Revision: D64014291

Pulled By: larryliu0820

fbshipit-source-id: 2757f2883d3f836e9efd45676f792c12f742e63d