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!");
+        }
+    }
+}

TestAssets/TestProjects/PortableTests/PortableAppWithIntentionalManagedDowngrade/project.json

@@ -0,0 +1,21 @@
+{
+  "compilationOptions": {
+    "emitEntryPoint": true
+  },
+  "dependencies": {},
+  "frameworks": {
+    "netstandard1.5": {
+      "imports": [
+        "dnxcore50",
+        "portable-net45+win8"
+      ],
+      "dependencies": {
+        "Microsoft.NETCore.App": {
+          "type": "platform",
+          "version": "1.0.0-rc2-23925"
+        },
+        "System.Linq": "4.0.0"
+      }
+    }
+  }
+}