[common] allow Dispatch 'alt' extractors to use custom URLs

gallery_dl/extractor/common.py

@@ -931,8 +931,11 @@ class Dispatch():
         }
 
         if alt is not None:
-            for sub, sub_alt in alt:
-                extractors[sub_alt] = extractors[sub]
+            for sub, sub_alt, url in alt:
+                if url is None:
+                    extractors[sub_alt] = extractors[sub]
+                else:
+                    extractors[sub_alt] = (extractors[sub][0], url)
 
         include = self.config("include", default) or ()
         if include == "all":

gallery_dl/extractor/pixiv.py

@@ -418,8 +418,8 @@ class PixivUserExtractor(Dispatch, PixivExtractor):
             (PixivNovelUserExtractor    , base + "novels"),
             (PixivSketchExtractor       , sketch),
         ), ("artworks",), (
-            ("bookmark", "novel-bookmark"),
-            ("user"    , "novel-user"),
+            ("bookmark", "novel-bookmark", None),
+            ("user"    , "novel-user"    , None),
         ))