Bazel: Switch to using bazlets

We prefer to consume maven_jar custom rule from bazlets repository, for
the same reasons as in Gerrit project:

* caching atifacts across different clones and projects
* exposing source classifiers and neverlink artifact

Change-Id: Ia560a68136186025926649f6985608fcf326082f

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
new file mode 100644
index 0000000..8b13789
--- /dev/null
+++ b/tools/BUILD
@@ -0,0 +1 @@
+