Implement IDisposable in ArchiveReaders

osu.Desktop/Beatmaps/IO/LegacyFilesystemReader.cs

@@ -48,5 +48,10 @@ namespace osu.Desktop.Beatmaps.IO
         public override BeatmapMetadata ReadMetadata()
         {
             return FirstMap.Metadata;
+        }
+        
+        public override void Dispose()
+        {
+            // no-op
         }
     }

osu.Game/Beatmaps/IO/ArchiveReader.cs

@@ -5,7 +5,7 @@ using osu.Framework.Platform;
 
 namespace osu.Game.Beatmaps.IO
 {
-    public abstract class ArchiveReader
+    public abstract class ArchiveReader : IDisposable
     {
         private class Reader
         {
@@ -41,6 +41,8 @@ namespace osu.Game.Beatmaps.IO
         /// <summary>
         /// Opens a stream for reading a specific file from this archive.
         /// </summary>
-        public abstract Stream ReadFile(string name);
+        public abstract Stream ReadFile(string name);
+
+        public abstract void Dispose();
     }
 }

osu.Game/Beatmaps/IO/OszArchiveReader.cs

@@ -58,5 +58,9 @@ namespace osu.Game.Beatmaps.IO
         {
             return FirstMap.Metadata;
         }
+
+        public override void Dispose()
+        {
+            Archive.Dispose();
         }
     }