update download system

优化资源文件下载流程。

Assets/YooAsset/Runtime/DownloadSystem/Downloader/DownloaderBase.cs

@@ -6,9 +6,12 @@ namespace YooAsset
 		protected enum ESteps
 		{
 			None,
-			CheckLocalFile,
-			CreateDownload,
+			CheckTempFile,
+			PrepareDownload,
+			CreateResumeDownloader,
+			CreateGeneralDownloader,
 			CheckDownload,
+			VerifyDownload,
 			TryAgain,
 			Succeed,
 			Failed,
@@ -56,7 +59,7 @@ namespace YooAsset
 			{
 				_failedTryAgain = failedTryAgain;
 				_timeout = timeout;
-				_steps = ESteps.CheckLocalFile;
+				_steps = ESteps.CheckTempFile;
 			}
 		}
 		public abstract void Update();