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/ProjectB/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 ProjectB
+    {
+        public static string GetMessage()
+        {
+            return "This string came from ProjectB";
+        }
+    }
+}

TestAssets/TestProjects/TestAppDependencyGraph/ProjectB/project.json

@@ -0,0 +1,26 @@
+{
+  "version": "1.0.0-*",
+  "buildOptions": {
+    "nowarn": [
+      "CS1591"
+    ],
+    "xmlDoc": true,
+    "additionalArguments": [
+      "-highentropyva+"
+    ]
+  },
+  "dependencies": {
+    "ProjectC": {
+      "target": "project",
+      "version": "1.0.0-*"
+    },
+    "ProjectD": {
+      "target": "project",
+      "version": "1.0.0-*"
+    },
+    "NETStandard.Library": "1.6.0"
+  },
+  "frameworks": {
+    "netstandard1.5": {}
+  }
+}