Move LinkCollector to a new collector.py module.

Author: cjerdonek

src/pip/_internal/collector.py

@@ -15,7 +15,7 @@
 
 if MYPY_CHECK_RUNNING:
     from typing import Optional, Text, Tuple, Union
-    from pip._internal.index import HTMLPage
+    from pip._internal.collector import HTMLPage
     from pip._internal.utils.hashes import Hashes