Add/remove solution items based on directory structure (#5197)

* WIP support solution folders for dotnet add and remove

* Add/remove solution folders based on directory hierarchy

* Fix tests

* Disable the solution building tests

* Address PR comments

* Fix a build break due to a new tool version used in the build

* Create SlnProjectExtensions and SlnProjectCollectionExtensions per PR comments

TestAssets/TestProjects/TestAppWithSlnAndCsprojInSubDirToRemove/src/Lib/Library.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Lib
+{
+    public class Library
+    {
+        public static string GetMessage()
+        {
+            return "Message from Lib";
+        }
+    }
+}