Make sure we never ship a versioned CoreFx ref assembly with the targeting pack

[wtgodbe]

There are a couple of CoreFx packages (e.g. System.IO.Pipelines) that are not in Microsoft.NetCore.App, but are in our shared framework. We should make sure we're guaranteeing that, when CoreFx ships new versions of those packages & we patch the targeting pack, we don't include the versioned ref assembly in our patched targeting pack (we should include the ref assembly that shipped with GA).

CC @dougbu @JunTaoLuo @ericstj

Context: dotnet/corefx#42837

[dougbu]

Remaining work is primarily testing.