CMake SDist only could skip sdist step

[henryiii]

Currently I don't think it's useful to use CMake only in the SDist only step, as if someone builds a wheel from source, the SDist step won't run. We should consider this use case, and see if there's a way (maybe with overrides) to add controls for Source vs. from SDist builds.

I think if.from-sdist would do it?