Unstatic GlobalFileSystem and rename it to FileSystem

Add a ModFiles field on ModData and move all access to the file system to go through that.

OpenRA.Game/MiniYaml.cs

@@ -12,7 +12,6 @@ using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
-using OpenRA.FileSystem;
 
 namespace OpenRA
 {
@@ -230,12 +229,6 @@ namespace OpenRA
 			return ret;
 		}
 
-		public static List<MiniYamlNode> FromFileInPackage(string path)
-		{
-			using (var stream = GlobalFileSystem.Open(path))
-				return FromLines(stream.ReadAllLines(), path);
-		}
-
 		public static Dictionary<string, MiniYaml> DictFromFile(string path)
 		{
 			return FromFile(path).ToDictionary(x => x.Key, x => x.Value);