Remove usage of Nuget.Packaging extension methods for IList.AddRange

osu.Game/Extensions/CollectionExtensions.cs

@@ -0,0 +1,22 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Generic;
+
+namespace osu.Game.Extensions
+{
+    public static class CollectionExtensions
+    {
+        public static void AddRange<T>(this ICollection<T> collection, IEnumerable<T> items)
+        {
+            // List<T> has a potentially more optimal path to adding a range.
+            if (collection is List<T> list)
+                list.AddRange(items);
+            else
+            {
+                foreach (T obj in items)
+                    collection.Add(obj);
+            }
+        }
+    }
+}