copy-plugin-deps: Copy Bazel dependency files from plugins

Copy external_plugin_deps.bzl into the Gerrit plugins directory
when present.

Also copy external_plugin_deps.MODULE.bazel when present, so
plugins using Bazel module dependency declarations are handled
the same way.

Update the role documentation accordingly.

Issue: 492561165
Change-Id: Id23dbe3b3fd24f63413e49d44ba3ef428e4faed0