More improvements to HLS/DASH external downloader code

* Fix error when there is no `protocol` in `info_dict`
* Move HLS byte range detection to `Aria2cFD` so that the download will fall back to the native downloader instead of ffmpeg
* Fix bug with getting no fragments in DASH
* Convert `check_results` in `can_download` to a generator

yt_dlp/downloader/common.py

@@ -326,6 +326,12 @@ class FileDownloader(object):
         """Report it was impossible to resume download."""
         self.to_screen('[download] Unable to resume')
 
+    @staticmethod
+    def supports_manifest(manifest):
+        """ Whether the downloader can download the fragments from the manifest.
+        Redefine in subclasses if needed. """
+        pass
+
     def download(self, filename, info_dict, subtitle=False):
         """Download to a filename using the info from info_dict
         Return True on success and False otherwise