[3.14] gh-136264: Fix ``--relative-paths`` for PEP 739's build-details.json (GH-138510) (#138638)

* gh-136264: Fix ``--relative-paths`` for PEP 739's build-details.json (GH-138510)

* KeyError is not raised for defaultdict
* Fix relative paths on different drives on Windows
* Add a round-trip test
(cherry picked from commit 057ee1741015067b46446e46eab467c1fa82610d)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Itamar Oren <itamarost@gmail.com>

* Update test_build_details.py

* Update Lib/test/test_build_details.py

---------

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Itamar Oren <itamarost@gmail.com>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>