Adapt to use of gerrit_plugin rule from bazlets for in-tree builds

Change-Id: I06a11ed73aca9fe0f66e7d6cf1e3162900bf9694

BUILD

diff --git a/BUILD b/BUILD
index b013399..0eb1b4c 100644
--- a/BUILD
+++ b/BUILD
@@ -1,4 +1,4 @@
-load("//tools/bzl:plugin.bzl", "gerrit_plugin")
+load("@com_googlesource_gerrit_bazlets//:gerrit_plugin.bzl", "gerrit_plugin")
 
 gerrit_plugin(
     name = "reviewnotes",