Allow migration of sln files (#4949)

* WIP Migrate sln files

* WIP add reference to redist

* Adding tests and fixing a few bugs

* Fix some tests

* Remove use of DeepClone

* Fix test build errors

* Fix more tests

TestAssets/TestProjects/TestAppWithSlnAndMultipleProjects/TestLibrary/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 Helper
+    {
+        public static string GetMessage()
+        {
+            return "This string came from the test library!";
+        }
+    }
+}