[3.13] gh-82916: Don't fail when importing from / with sys.pycache_prefix set (GH-30456) (GH-137905)

(cherry picked from commit d8a9466e29f59f81aacf1959b835862bcd47b731)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>