update cache system

优化缓存系统的存储目录结构。

Assets/YooAsset/Runtime/CacheSystem/CacheSystem.cs

@@ -23,6 +23,15 @@ namespace YooAsset
 			_cachedDic.Clear();
 		}
 
+		/// <summary>
+		/// 获取缓存文件总数
+		/// </summary>
+		public static int GetCachedFilesCount(string packageName)
+		{
+			var cache = GetOrCreateCache(packageName);
+			return cache.GetCachedFilesCount();
+		}
+
 		/// <summary>
 		/// 查询是否为验证文件
 		/// </summary>