add trimming of published output based on sharedfx

Any dependencies which **exactly** match the version requested in the
graph originating at the `type: platform` dependency (if any) are
trimmed from the publish output

TestAssets/TestProjects/PortableTests/PortableAppWithIntentionalManagedDowngrade/Program.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace PortableApp
+{
+    public static class Program
+    {
+        public static void Main(string[] args)
+        {
+            Console.WriteLine("Hello, World!");
+        }
+    }
+}