[youtube:playlist] Use an iterator for the entries (closes #5935)

So that '--playlist-end' downloads only the required pages.

youtube_dl/extractor/youtube.py

@@ -1290,7 +1290,6 @@ class YoutubePlaylistIE(YoutubeBaseInfoExtractor):
     def _extract_playlist(self, playlist_id):
         url = self._TEMPLATE_URL % playlist_id
         page = self._download_webpage(url, playlist_id)
-        more_widget_html = content_html = page
 
         for match in re.findall(r'<div class="yt-alert-message">([^<]+)</div>', page):
             match = match.strip()
@@ -1310,14 +1309,15 @@ class YoutubePlaylistIE(YoutubeBaseInfoExtractor):
                 self.report_warning('Youtube gives an alert message: ' + match)
 
         # Extract the video ids from the playlist pages
-        ids = []
-
+        def _entries():
+            more_widget_html = content_html = page
             for page_num in itertools.count(1):
                 matches = re.finditer(self._VIDEO_RE, content_html)
                 # We remove the duplicates and the link with index 0
                 # (it's not the first video of the playlist)
                 new_ids = orderedSet(m.group('id') for m in matches if m.group('index') != '0')
-            ids.extend(new_ids)
+                for vid_id in new_ids:
+                    yield self.url_result(vid_id, 'Youtube', video_id=vid_id)
 
                 mobj = re.search(r'data-uix-load-more-href="/?(?P<more>[^"]+)"', more_widget_html)
                 if not mobj:
@@ -1338,8 +1338,7 @@ class YoutubePlaylistIE(YoutubeBaseInfoExtractor):
             r'(?s)<h1 class="pl-header-title[^"]*">\s*(.*?)\s*</h1>',
             page, 'title')
 
-        url_results = self._ids_to_results(ids)
-        return self.playlist_result(url_results, playlist_id, playlist_title)
+        return self.playlist_result(_entries(), playlist_id, playlist_title)
 
     def _real_extract(self, url):
         # Extract playlist id