Migrate P2P references

- Given a project migrate all the project references recursively.
- Add a boolean option '--skip-project-references' to skip migrating P2P references. By default P2P references are migrated.
- Remove the '--output' since it is not used heavily and makes migrating multiple projects easier.
- Add tests

TestAssets/TestProjects/TestAppDependencyGraph/ProjectC/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 ProjectC";
+        }
+    }
+}