Move ArchiveReaders to a more global namespace

Also moves delete and action logic to a shared implementation

osu.Game/IO/Archives/ZipArchiveReader.cs

@@ -0,0 +1,50 @@
+// 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 Ionic.Zip;
+
+namespace osu.Game.IO.Archives
+{
+    public sealed class ZipArchiveReader : ArchiveReader
+    {
+        private readonly Stream archiveStream;
+        private readonly ZipFile archive;
+
+        public ZipArchiveReader(Stream archiveStream, string name = null)
+            : base(name)
+        {
+            this.archiveStream = archiveStream;
+            archive = ZipFile.Read(archiveStream);
+        }
+
+        public override Stream GetStream(string name)
+        {
+            ZipEntry entry = archive.Entries.SingleOrDefault(e => e.FileName == name);
+            if (entry == null)
+                throw new FileNotFoundException();
+
+            // allow seeking
+            MemoryStream copy = new MemoryStream();
+
+            using (Stream s = entry.OpenReader())
+                s.CopyTo(copy);
+
+            copy.Position = 0;
+
+            return copy;
+        }
+
+        public override void Dispose()
+        {
+            archive.Dispose();
+            archiveStream.Dispose();
+        }
+
+        public override IEnumerable<string> Filenames => archive.Entries.Select(e => e.FileName).ToArray();
+
+        public override Stream GetUnderlyingStream() => archiveStream;
+    }
+}