refactor: `get_poetry_deps` leverages `common.PoetryPackage`

[souliane]

Also add extras field to common.PoetryPackage. This allows to clean update_or_remove_additional_deps in sync_hooks_additional_dependencies.py

Successful PR Checklist:

• [ ] Tests
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• [ ] Documentation
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PR label(s):

• [ ] https://github.com/ewjoachim/poetry-to-pre-commit/labels/breaking
• [ ] https://github.com/ewjoachim/poetry-to-pre-commit/labels/feature
• [ ] https://github.com/ewjoachim/poetry-to-pre-commit/labels/bugfix
• [x] https://github.com/ewjoachim/poetry-to-pre-commit/labels/miscellaneous
• [ ] https://github.com/ewjoachim/poetry-to-pre-commit/labels/dependencies
• [ ] https://github.com/ewjoachim/poetry-to-pre-commit/labels/documentation