build_target_util: Rename to build_target_lib.

Nitty name change. The module contains a class and is in lib, so
should be a _lib, not a _util.

BUG=chromium:1060351
TEST=run_tests

Change-Id: I539dc8e5288e71cc9408bc68861f20579ac70937
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2095746
Commit-Queue: Alex Klein <saklein@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/api/controller/controller_util.py b/api/controller/controller_util.py
index 3b1bf52..3199525 100644
--- a/api/controller/controller_util.py
+++ b/api/controller/controller_util.py
@@ -13,7 +13,7 @@
 from chromite.cbuildbot import goma_util
 from chromite.lib import constants
 from chromite.lib import portage_util
-from chromite.lib.build_target_util import BuildTarget
+from chromite.lib.build_target_lib import BuildTarget
 from chromite.lib.chroot_lib import Chroot