[blouirts123]

norm_and_check() accepts a relative backend directory whose symlink target is outside the source tree. This lets a project unintentionally depend on an external directory despite the helper's in-tree path check.

Resolve both the source root and requested backend path for containment checks, while continuing to return the original normalized absolute path. This preserves valid in-tree links and source roots accessed through a link. It follows PEP 517's recommended frontend check for self-contained source trees: https://peps.python.org/pep-0517/#in-tree-build-backends .

Validation:

• All seven configured Nox test environments (CPython 3.8–3.13 and PyPy 3.11) produced 41 passed, 3 new symlink tests skipped for Windows permissions, and one existing WinError 1314 failure in test_path_pollution_symlinked_in_process. That failure also reproduces on the unchanged base.
• A paired native Windows junction probe accepted/loaded the outside backend on base and rejected it with the change; inside targets and a linked source root remain accepted.
• Unchanged Nox lint passed on Python 3.10, including mypy and all pre-commit hooks. Sphinx docs with -W passed.
• Independent final review and git diff --check passed. Linux/macOS tests were not run locally.