Just hardcode to delete the directory after unpacking it.

pip/req/req_set.py

@@ -141,6 +141,16 @@ def __init__(self, build_dir, src_dir, download_dir, upgrade=False,
                  ignore_dependencies=False, force_reinstall=False,
                  use_user_site=False, session=None, pycompile=True,
                  isolated=False, wheel_download_dir=None):
+        """Create a RequirementSet.
+
+        :param wheel_download_dir: Where still-packed .whl files should be
+            written to. If None they are written to the download_dir parameter.
+            Separate to download_dir to permit only keeping wheel archives for
+            pip wheel.
+        :param download_dir: Where still packed archives should be written to.
+            If None they are not saved, and are deleted immediately after
+            unpacking.
+        """
         if session is None:
             raise TypeError(
                 "RequirementSet() missing 1 required keyword argument: "
@@ -437,13 +447,11 @@ 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(