runtime: System.ComponentModel.Composition doesn't work on UWP

ML.NET has recently started using System.ComponentModel.Composition to enable some extensiblity scenarios. However, using that is now breaking ML.NET on UWP apps. See: https://github.com/dotnet/machinelearning/issues/1595.

I looked into this and it appears that https://www.nuget.org/packages/System.ComponentModel.Composition/ has a placholder (_._) for lib/uap10.0.16299. However, that assembly doesn’t appear to ship inbox on UWP.

So something should change here. Either we should ship that assembly inbox in UWP, or we should remove the placeholder in the package.

/cc @ericstj @joperezr @danmosemsft