Support recursive extras defined in pyproject.toml

[masenf]

With this change, tox will now expand self-referential extras discovered in package deps to respect local modifications to package metadata. This allows a package extra to explicitly depend on another package extra, which previously only worked with non-static metadata.

Fix #2904

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• ran the linter to address style issues (tox -e fix)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in docs/changelog folder
• updated/extended the documentation

[masenf]

Thought about this implementation a bit and it could get stuck in an infinite loop if there is a circular extras chain. Need up update the code to only visit each extra once.

Edit: latest commit avoids the infinite loop and adds a test for it; not sure if i should also add an escape hatch to the test? Currently it just gets stuck in an infinite loop and needs to be interrupted.

[masenf]

Pushed another test case for when the package deps themselves reference an extra, and this currently fails. So I'm going to fix that as well and unify the extra unroll routines in tox/src/tox/tox_env/python/virtual_env/package/util.py so both metadata and pyproject can use the same algorithm.