MultiProjectValidator Fixes

Ignore desktop frameworks (if specified)

tools/MultiProjectValidator/ProjectContextExtensions.cs

@@ -0,0 +1,17 @@
+using System.IO;
+using System.Collections.Generic;
+using Microsoft.DotNet.ProjectModel;
+
+namespace MultiProjectValidator
+{
+    public static class ProjectContextExtensions
+    {
+        private static readonly string s_desktopTfmPrefix = ".NETFramework";
+        public static bool TargetIsDesktop(this ProjectContext context)
+        {
+            var targetFramework = context.TargetFramework.ToString();
+
+            return targetFramework.StartsWith(s_desktopTfmPrefix);
+        }
+    }
+}