Fix inverted only-download in req_set.

only_download means 'mark the extracted tree for automatic deletion'.
is_download in req_set indicates we're downloading to a user specified
directory - and has nothing to do with deleting the extracted tree.
The actual thing we want to use to control that is the deprecated
--no-install parameter.

pip/req/req_set.py

@@ -446,13 +446,11 @@ def _prepare_file(self, finder, req_to_install):
                             self.wheel_download_dir:
                         # when doing 'pip wheel`
                         download_dir = self.wheel_download_dir
-                        do_download = True
                     else:
                         download_dir = self.download_dir
-                        do_download = self.is_download
                     unpack_url(
                         req_to_install.link, req_to_install.source_dir,
-                        download_dir, do_download, session=self.session,
+                        download_dir, True, session=self.session,
                     )
                 except requests.HTTPError as exc:
                     logger.critical(