PublicWinMDReferences wants the path to the winmd file. For APIs that are part of the Windows 10 SDK there’s a helper function that’ll get you the path to the latest version based on the contract name. So something like:
PublicWinMDReferences.Add(VCEnvironment.GetLatestMetadataPathForApiContract("Windows.Foundation.UniversalApiContract"));
It’s only relevant for UWP targets though, so you’ll still need to make sure that nothing’s trying to build your plugin for Windows desktop.