refactor the runtime code

重构了运行时代码，支持全新的文件系统。

Assets/YooAsset/Runtime/DownloadSystem/DownloadSystemHelper.cs

@@ -0,0 +1,50 @@
+using UnityEngine.Networking;
+
+namespace YooAsset
+{
+    /// <summary>
+    /// 自定义下载器的请求委托
+    /// </summary>
+    public delegate UnityWebRequest UnityWebRequestDelegate(string url);
+
+    internal class DownloadSystemHelper
+    {
+        public static UnityWebRequestDelegate UnityWebRequestCreater = null;
+        public static UnityWebRequest NewUnityWebRequestGet(string requestURL)
+        {
+            UnityWebRequest webRequest;
+            if (UnityWebRequestCreater != null)
+                webRequest = UnityWebRequestCreater.Invoke(requestURL);
+            else
+                webRequest = new UnityWebRequest(requestURL, UnityWebRequest.kHttpVerbGET);
+            return webRequest;
+        }
+
+        /// <summary>
+        /// 获取WWW加载本地资源的路径
+        /// </summary>
+        public static string ConvertToWWWPath(string path)
+        {
+#if UNITY_EDITOR
+            return StringUtility.Format("file:///{0}", path);
+#elif UNITY_WEBGL
+            return path;
+#elif UNITY_IPHONE
+            return StringUtility.Format("file://{0}", path);
+#elif UNITY_ANDROID
+            if (path.StartsWith("jar:file://"))
+                return path;
+            else
+                return StringUtility.Format("jar:file://{0}", path);
+#elif UNITY_STANDALONE_OSX
+            return new System.Uri(path).ToString();
+#elif UNITY_STANDALONE
+            return StringUtility.Format("file:///{0}", path);
+#elif UNITY_OPENHARMONY
+            return path;
+#else
+            return path;
+#endif
+        }
+    }
+}