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

Change-Id: I0c74595ec55f4c564bd608474c7029978d546373

BUILD

diff --git a/BUILD b/BUILD
index 3473bbe..25f726a 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 = "commit-message-length-validator",