Fix P2P migration

Fixes #4269 by deep cloning the MsbuildTemplateProject for each P2P reference. Add test which migrates and builds P2P references.

TestAssets/TestProjects/TestAppDependencyGraph/CsprojLibrary2/Helper.cs

@@ -0,0 +1,15 @@
+// Copyright (c) .NET Foundation and contributors. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+using System;
+
+namespace TestLibrary
+{
+    public static class ProjectC
+    {
+        public static string GetMessage()
+        {
+            return "This string came from CsprojLibrary2";
+        }
+    }
+}