fix(inspect): ignore non-package top-level dirs in sdist site_paths - #372

Summary

After #364 (840b79e), _resolve_top_level_paths keeps every file in namespace directories (so data files in wheels like PyQt5-Qt5's PyQt5/Qt5/qml/** are linked in venvs). However, sdist inspection feeds _resolve_top_level_paths every top-level directory in the sdist, causing non-package top-level directories without any Python modules (such as .github/, requirements/, or licenses/) to resolve to file lists and create dangling symlinks under venvs_site_packages=yes.

Filter top_level_dirs in _find_site_paths_from_items (sdist path only) to valid Python identifiers that contain at least one Python module (_extract_module_name) at any depth. Both top_level.txt and wheel inspection (_find_wheel_paths) remain unchanged, and data files inside sdist namespace directories that also contain Python modules are still preserved.

Test Plan

• Added unit tests in tests/unit/inspect_package_test.py (test_sdist_ignores_non_package_top_level_dirs and test_sdist_namespace_keeps_data_files_when_python_present).
• Verified tests/e2e/requirements (packaging-23.2 sdist site_paths returns ["noxfile.py", "packaging", "tasks"]) and tests/e2e/namespace_pkgs.