Add LegacyFileArchiveReader

Treats files as "archives" for import.

osu.Game/Database/ArchiveModelManager.cs

@@ -486,6 +486,8 @@ namespace osu.Game.Database
                 return new ZipArchiveReader(File.Open(path, FileMode.Open, FileAccess.Read, FileShare.Read), Path.GetFileName(path));
             if (Directory.Exists(path))
                 return new LegacyDirectoryArchiveReader(path);
+            if (File.Exists(path))
+                return new LegacyFileArchiveReader(path);
             throw new InvalidFormatException($"{path} is not a valid archive");
         }
     }