chromite: Move tree_status.py to chromite/lib

BUG=chromium:729200
TEST=FEATURES=test emerge chromite

Change-Id: I51c3ca2b8bc73017694296e88779e69e5111abdf
Reviewed-on: https://chromium-review.googlesource.com/528018
Commit-Ready: Drew Davenport <ddavenport@chromium.org>
Tested-by: Drew Davenport <ddavenport@chromium.org>
Reviewed-by: Drew Davenport <ddavenport@chromium.org>
diff --git a/scripts/cbuildbot.py b/scripts/cbuildbot.py
index 8bc55ae..eaed29c 100644
--- a/scripts/cbuildbot.py
+++ b/scripts/cbuildbot.py
@@ -25,7 +25,6 @@
 from chromite.cbuildbot import repository
 from chromite.cbuildbot import tee
 from chromite.cbuildbot import topology
-from chromite.cbuildbot import tree_status
 from chromite.cbuildbot import trybot_patch_pool
 from chromite.cbuildbot.stages import completion_stages
 from chromite.lib import builder_status_lib
@@ -46,6 +45,7 @@
 from chromite.lib import retry_stats
 from chromite.lib import sudo
 from chromite.lib import timeout_util
+from chromite.lib import tree_status
 from chromite.lib import ts_mon_config