Enumerate pip's vendored dependencies in SBOM

[sethmlarson]

Bug report

Bug description:

Part of #112302

Currently pip's package entry in the SBOM is quite simple, including only itself but not all the vendored projects and their licenses/IDs. See comment from @hroncok on DPO on why this is problematic: https://discuss.python.org/t/create-and-distribute-software-bill-of-materials-sbom-for-python-artifacts/39293/24

My proposed changes to generate_sbom.py is the following:

• Find the entry pip/_vendor/vendor.txt in the pip wheel archive.
• Read the content, parse the requirements into names and versions.
• Ensure all entries are represented as packages in the SBOM with a pip DEPENDS_ON <package> relationship.

This approach lets the license identifiers be specified in the SBOM like other packages but then would raise an error if pip is upgraded with a difference in vendored dependencies or versions allowing the reviewer to acknowledge any changes.

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-114250: Fetch metadata for pip and its vendored dependencies from PyPI #114450