Project.json schema warnings don't get displayed

When building a project.json that has schema warnings (and other warnings), we are not writing the warnings to the console. This is a regression.

The fix is to add all diagnostic messages to the LibraryManager, which is responsible to hold all the diagnostic messages.

Fix 3021

TestAssets/TestProjects/TestLibraryWithDeprecatedProjectFile/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 void SayHi()
+        {
+            Console.WriteLine("Hello there!");
+        }
+    }
+}