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!");
+        }
+    }
+}

TestAssets/TestProjects/TestLibraryWithDeprecatedProjectFile/project.json

@@ -0,0 +1,13 @@
+{
+  "version": "1.0.0-*",
+  "compilationOptions": {
+    "xmlDoc": true
+  },
+  "packInclude": {},
+  "dependencies": {
+    "NETStandard.Library": "1.5.0-rc2-24027"
+  },
+  "frameworks": {
+    "netstandard1.5": {}
+  }
+}