Fast checking for file existance

OpenRA.FileFormats/Package.cs

@@ -18,6 +18,7 @@ namespace OpenRA.FileFormats
 	public interface IFolder
 	{
 		Stream GetContent(string filename);
+		bool Exists(string filename);
 		IEnumerable<uint> AllFileHashes();
 	}
 
@@ -143,6 +144,11 @@ namespace OpenRA.FileFormats
 		{
 			return index.Keys;
 		}
+		
+		public bool Exists(string filename)
+		{
+			return index.ContainsKey(PackageEntry.HashFilename(filename));
+		}
 	}
 
 	[Flags]