gh-134632: Fix `build-details.json` to use `INCLUDEPY` path (#134633)

* gh-134632: Fix `build-details.json` to use `INCLUDEPY` path

Fix ``build-details.json`` generation to use ``INCLUDEPY``, in order to
reference the ``pythonX.Y`` subdirectory of the include directory, as
required in :pep:`739`, instead of the top-level include directory.

* test_build_details: Add tests for the c_api section

* test_build_details: Expect pkgconfig for CPython unconditionally