move from map builtin to list comprehensions

The Python style guide says to avoid map() and stick to list
comprehensions.  Python 3 also changes its to be a generator,
so now is a good time to replace all of its users in the tree.

BUG=chromium:980619
TEST=lint is unchanged in chromite
TEST=unittests pass

Change-Id: I98dfc6e6b1107f8926a402006bba3cd287506165
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1716014
Reviewed-by: LaMont Jones <lamontjones@chromium.org>
Reviewed-by: Mike Nichols <mikenichols@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/api/controller/artifacts.py b/api/controller/artifacts.py
index 88444cc..60fe1bc 100644
--- a/api/controller/artifacts.py
+++ b/api/controller/artifacts.py
@@ -7,7 +7,6 @@
 
 from __future__ import print_function
 
-import functools
 import os
 
 from chromite.api import validate
@@ -75,7 +74,7 @@
   img_types = [constants.IMAGE_TYPE_TEST, constants.IMAGE_TYPE_DEV,
                constants.IMAGE_TYPE_BASE]
   img_names = [constants.IMAGE_TYPE_TO_NAME[t] for t in img_types]
-  img_paths = map(functools.partial(os.path.join, img_dir), img_names)
+  img_paths = [os.path.join(img_dir, x) for x in img_names]
   valid_images = [x for x in img_paths if os.path.exists(x)]
 
   if not valid_images: