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

test/dotnet-build.Tests/BuildWarningsTests.cs

@@ -0,0 +1,27 @@
+// 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 FluentAssertions;
+using Microsoft.DotNet.Tools.Test.Utilities;
+using Xunit;
+
+namespace Microsoft.DotNet.Tools.Builder.Tests
+{
+    public class BuildWarningsTests : TestBase
+    {
+        [Fact]
+        public void HavingDeprecatedProjectFileProducesWarning()
+        {
+            var testInstance = TestAssetsManager.CreateTestInstance("TestLibraryWithDeprecatedProjectFile").WithLockFiles();
+
+            new BuildCommand(testInstance.TestRoot)
+                .ExecuteWithCapturedOutput()
+                .Should()
+                .Pass()
+                .And
+                .HaveStdErrContaining("DOTNET1015: The 'compilationOptions' option is deprecated. Use 'buildOptions' instead.")
+                .And
+                .HaveStdErrContaining("DOTNET1015: The 'packInclude' option is deprecated. Use 'files' in 'packOptions' instead.");
+        }
+    }
+}