Fix package validation (reference closure check) to work with nonlocal NETCoreApp

[dagood]

windowsdesktop/pkg/windowsdesktop/pkg/Microsoft.WindowsDesktop.App.pkgproj

Lines 20 to 27 in 6c6432c

	<!--
	Turn off package validation: while in Core-Setup, this depended on extracting the list of
	NETCoreApp DLLs from netcoreapp.depproj. We need to figure out how to implement this based on
	a restored package, instead. (Should be somewhat simple: a target that depends on NuGet's
	content restoration target that filters found assets based on origin package and puts them in
	the list of ignored references.)
	-->
	<SkipValidatePackage>true</SkipValidatePackage>

[...] while in Core-Setup, this depended on extracting the list of NETCoreApp DLLs from netcoreapp.depproj. We need to figure out how to implement this based on a restored package, instead. (Should be somewhat simple: a target that depends on NuGet's content restoration target that filters found assets based on origin package and puts them in the list of ignored references.)

This check is important to get running because:

[...] the WPF/WinForms packages don't declare their transitive dependencies and the depproj has to fill it in correctly. Being wrong means dependency failures at runtime.