Update resource stores with GetAvailableResources

osu.Game/IO/Archives/ArchiveReader.cs

@@ -15,6 +15,8 @@ namespace osu.Game.IO.Archives
         /// </summary>
         public abstract Stream GetStream(string name);
 
+        public IEnumerable<string> GetAvailableResources() => Filenames;
+
         public abstract void Dispose();
 
         /// <summary>

osu.Game/Skinning/LegacySkin.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
@@ -140,6 +141,8 @@ namespace osu.Game.Skinning
                 return path == null ? null : underlyingStore.GetStream(path);
             }
 
+            public IEnumerable<string> GetAvailableResources() => source.Files.Select(f => f.Filename);
+
             byte[] IResourceStore<byte[]>.Get(string name) => GetAsync(name).Result;
 
             public Task<byte[]> GetAsync(string name)