Use world-container when finding players to convert. Fixes BUKKIT-5530

src/main/java/net/minecraft/server/NameReferencingFileConverter.java

@@ -373,7 +373,7 @@ public class NameReferencingFileConverter {
 
     private static File d(PropertyManager propertymanager) {
         String s = propertymanager.getString("level-name", "world");
-        File file1 = new File(s);
+        File file1 = new File(MinecraftServer.getServer().server.getWorldContainer(), s); // CraftBukkit - Respect container setting
 
         return new File(file1, "players");
     }