Fix transitive load of GWT_PLUGIN_DEPS for in-tree plugin builds

If any plugin that depends on GWT_PLUGIN_DEPS (e.g. importer, reviewers)
is included in the gerrit in-tree build, running the command:

  bazel test //...

fails with:

  ERROR: gerrit/plugins/importer/tools/eclipse/BUILD:2:1:
  file '//tools/bzl:plugin.bzl' does not contain symbol 'GWT_PLUGIN_DEPS'

Bug: Issue 10855
Change-Id: I90ca42e0425d3749525e27b3c876de2fee1221ab
diff --git a/tools/bzl/plugin.bzl b/tools/bzl/plugin.bzl
index 84ec1c1..eb25007 100644
--- a/tools/bzl/plugin.bzl
+++ b/tools/bzl/plugin.bzl
@@ -3,12 +3,13 @@
     "//tools/bzl:gwt.bzl",
     "GWT_COMPILER_ARGS",
     "GWT_JVM_ARGS",
-    "GWT_PLUGIN_DEPS",
     "GWT_PLUGIN_DEPS_NEVERLINK",
     "GWT_TRANSITIVE_DEPS",
     "gwt_binary",
+    _gwt_plugin_deps = "GWT_PLUGIN_DEPS",
 )
 
+GWT_PLUGIN_DEPS = _gwt_plugin_deps
 PLUGIN_DEPS = ["//gerrit-plugin-api:lib"]
 PLUGIN_DEPS_NEVERLINK = ["//gerrit-plugin-api:lib-neverlink"]