gh-156404: Restore the 'fullname' variable for .pth import lines (#156406)
PEP 829 moved .pth handling from site.addpackage() into
StartupState._exec_imports(), and the local holding the path of the
.pth file being processed was renamed from 'fullname' to 'filename'.
Import lines in .pth files generated before Python 3.15 read that
local, so they now fail with NameError on every interpreter startup.
Inject 'fullname' into the frame which executes pth code, next to the
'sitedir' shim added for gh-149671.
Reported downstream as https://github.com/karellen/wheel-axle/issues/38
Co-authored-by: Barry Warsaw <barry@python.org>