refactor: add index.bzl

The build_bazel_rules_nodejs workspace is the last one using defs.bzl

see #1068

examples/webapp/WORKSPACE

diff --git a/examples/webapp/WORKSPACE b/examples/webapp/WORKSPACE
index 6581ce8..d6d20e4 100644
--- a/examples/webapp/WORKSPACE
+++ b/examples/webapp/WORKSPACE
@@ -25,7 +25,7 @@
     urls = ["https://github.com/bazelbuild/rules_nodejs/releases/download/0.37.1/rules_nodejs-0.37.1.tar.gz"],
 )
 
-load("@build_bazel_rules_nodejs//:defs.bzl", "yarn_install")
+load("@build_bazel_rules_nodejs//:index.bzl", "yarn_install")
 
 yarn_install(
     name = "npm",