Provide a way to leverage incremental from pyproject.toml without importing the target package

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From discussion on twisted/treq#388 —

I don't see anything here equivalent to the use_incremental=True argument to setup(). It looks like we need to add support to incremental for direct integration with setuptools, like setuptools_scm has: https://github.com/pypa/setuptools_scm/blob/102f56a896bc4ea8d4c58d9c49f455f15310de38/pyproject.toml#L76-L77

The way that we presently allow the ability to have incremental eval a module without a full import is only via the distutils.setup_keywords entrypoint, which is deprecated and heavily discouraged as it doesn't work with pyproject.toml.

In Treq, and potentially in other projects, importing the package itself loads in a bunch of the package's submodules, which then pulls in dependent packages, so to get Treq off of setup.py, we would need to have its build dependencies fully replicate its runtime dependencies, which is obviously not ideal.