Depend on .NET Framework targeting pack nugets

Depend on .NET Framework reference assemblies via the new nuget
packages. This allows us to build .NET Framework on systems where
.NET Framework isn't installed (e.g. Linux).

Author: roji

Directory.Build.props

@@ -0,0 +1,9 @@
+<Project>
+
+  <!-- Reference .NET Framework reference assemblies, allows building on environments without .NET Framework installed
+       (e.g. Linux). Gets ignored on non-framework TFMs. -->
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NETFramework.ReferenceAssemblies" Version="1.0.0-preview.1" PrivateAssets="All" />
+  </ItemGroup>
+
+</Project>