Add missing pyproject_hooks to DEBUNDLED preload list

Otherwise, tests fail to load:

```
ImportError while loading conftest '/tmp/portage/dev-python/pip-24.1-r1/work/pip-24.1/tests/conftest.py'.
tests/conftest.py:49: in <module>
    from pip._internal.utils.temp_dir import global_tempdir_manager
../pip-24.1-python3_13/install/usr/lib/python3.13/site-packages/pip/_internal/utils/temp_dir.py:20: in <module>
    from pip._internal.utils.misc import enum, rmtree
../pip-24.1-python3_13/install/usr/lib/python3.13/site-packages/pip/_internal/utils/misc.py:38: in <module>
    from pip._vendor.pyproject_hooks import BuildBackendHookCaller
E   ModuleNotFoundError: No module named 'pip._vendor.pyproject_hooks'
```

Author: mgorny

src/pip/_vendor/__init__.py

@@ -69,6 +69,7 @@ def vendored(modulename):
     vendored("pkg_resources")
     vendored("platformdirs")
     vendored("progress")
+    vendored("pyproject_hooks")
     vendored("requests")
     vendored("requests.exceptions")
     vendored("requests.packages")