Move ArchiveReaders to a more global namespace

Also moves delete and action logic to a shared implementation

osu.Game/IO/Archives/LegacyFilesystemReader.cs

@@ -0,0 +1,34 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using osu.Framework.IO.File;
+
+namespace osu.Game.IO.Archives
+{
+    /// <summary>
+    /// Reads an extracted legacy beatmap from disk.
+    /// </summary>
+    public class LegacyFilesystemReader : ArchiveReader
+    {
+        private readonly string path;
+
+        public LegacyFilesystemReader(string path) : base(Path.GetFileName(path))
+        {
+            this.path = path;
+        }
+
+        public override Stream GetStream(string name) => File.OpenRead(Path.Combine(path, name));
+
+        public override void Dispose()
+        {
+            // no-op
+        }
+
+        public override IEnumerable<string> Filenames => Directory.GetFiles(path, "*", SearchOption.AllDirectories).Select(f => FileSafety.GetRelativePath(f, path)).ToArray();
+
+        public override Stream GetUnderlyingStream() => null;
+    }
+}