Reorganize PlayerReference loading to enable upgrade rules for them.

The PlayerReference definitions are now stored
as raw MiniYamlNodes in the map.

OpenRA.Mods.Common/UtilityCommands/UpgradeMapCommand.cs

@@ -30,6 +30,7 @@ namespace OpenRA.Mods.Common.UtilityCommands
 
 			UpgradeRules.UpgradeWeaponRules(engineDate, ref map.WeaponDefinitions, null, 0);
 			UpgradeRules.UpgradeActorRules(engineDate, ref map.RuleDefinitions, null, 0);
+			UpgradeRules.UpgradePlayers(engineDate, ref map.PlayerDefinitions, null, 0);
 			map.Save(args[1]);
 		}
 	}