build: Use maven_jar from bazlets/core

Newer Bazel releases have a different maven_jar rule than the one
provided by bazlets/core which can lead to different and undesired
behavior.

Change-Id: I724ed8677fc3645c9bbb91cdb315a9ee03c833ee

external_plugin_deps.bzl

diff --git a/external_plugin_deps.bzl b/external_plugin_deps.bzl
index 901cd0f..f33e277 100644
--- a/external_plugin_deps.bzl
+++ b/external_plugin_deps.bzl
@@ -1,4 +1,4 @@
-load("@bazel_tools//tools/build_defs/repo:maven_rules.bzl", "maven_jar")
+load("//tools/bzl:maven_jar.bzl", "maven_jar")
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 
 def external_plugin_deps():