Today we are using ReferenceFromRuntime items to reference System.Private.CoreLib. This requires the developer to specify the RuntimeConfiguration when building the product and even when building test projects.

We could leverage the TargetFramework.Sdk for System.Private.CoreLib to multiplex on different OSs as we do for libraries projects or avoid that by making sure that S.P.CoreLib's surface area is identical for different OSs (which AFAIK should be case now with the WinRT removal).

This is high on our developer productivity radar as developers need to build the clr.corelib subset today explicitly before being able to build the libraries source projects.

Related: #36464

cc @ericstj @safern @Anipik