Allow plugins to contribute external workspace deps

When building a Gerrit plugin in-tree, allow to add extra dependencies
to the WORKSPACE file.

The only operation required will be the overwrite of the
plugins/external_plugin_deps.bzl file with the one provided by the
plugin.

Bug: Issue 5305
Change-Id: I07e4538f9f81ef923a4ca1aeceaee75d8429434e
diff --git a/WORKSPACE b/WORKSPACE
index 3329460..5233b0d 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -2,6 +2,7 @@
 
 load("//tools/bzl:maven_jar.bzl", "maven_jar", "GERRIT", "MAVEN_LOCAL")
 load("//lib/codemirror:cm.bzl", "CM_VERSION", "DIFF_MATCH_PATCH_VERSION")
+load("//plugins:external_plugin_deps.bzl", "external_plugin_deps")
 
 ANTLR_VERS = "3.5.2"
 
@@ -1104,3 +1105,4 @@
 load("//lib/js:bower_archives.bzl", "load_bower_archives")
 
 load_bower_archives()
+external_plugin_deps()