use extras as feature flags for setup_requires

[ghost]

Originally reported by: RonnyPfannschmidt (Bitbucket: RonnyPfannschmidt, GitHub: RonnyPfannschmidt)

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i have a package that provides file finders among other functionalities,
i want users to be able to opt into the file finders

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/347

[jaraco]

setup_requires is deprecated (#2823). Any feature like this will need to be developed into the packaging standards. I'd recommend instead to implement settings in pyproject.toml that the package could honor.