Honor MANIFEST.in exclusions in strict editable installs

[goutamadwant]

Summary of changes

Strict editable installs currently build their link tree from every discovered Python module, including modules excluded by MANIFEST.in. This can make an excluded module importable from a strict editable install even though it is absent from the source distribution and the wheel built from it.

This change filters Python module mappings through the generated egg-info manifest only when strict editable mode is selected. Manifest and module paths are normalized before comparison so the filtering also handles platform-specific path separators. Other editable modes and regular wheel builds keep their existing behavior.

The regression test excludes a package module in MANIFEST.in, performs a strict editable install, and verifies that the module cannot be discovered. A bugfix news fragment is included.

Closes #5089

Testing

• pytest setuptools/tests/test_editable_install.py::TestLinkTree setuptools/tests/test_build_py.py -p no:cov
• pytest setuptools/tests/test_editable_install.py -p no:cov
• ruff format --check setuptools/command/build_py.py setuptools/command/editable_wheel.py setuptools/tests/test_editable_install.py
• ruff check setuptools/command/build_py.py setuptools/command/editable_wheel.py setuptools/tests/test_editable_install.py

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.

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