Replace @npm_bazel_rollup with @npm//@bazel/rollup

This is needed after we have rules_nodejs upgraded to 2.0

Change-Id: Ib1d2a221308fe7de6f71220299fd0f767757b6e5

BUILD

diff --git a/BUILD b/BUILD
index 3ff7216..4dceb11 100644
--- a/BUILD
+++ b/BUILD
@@ -1,5 +1,5 @@
 load("//tools/bzl:js.bzl", "polygerrit_plugin")
-load("@npm_bazel_rollup//:index.bzl", "rollup_bundle")
+load("@npm//@bazel/rollup:index.bzl", "rollup_bundle")
 
 
 polygerrit_plugin(
@@ -20,4 +20,4 @@
     deps = [
         "@tools_npm//rollup-plugin-node-resolve",
     ],
-)
\ No newline at end of file
+)