SonataUserBundle : composer.json (dev-master), & configuration (except the ApplicationSonataUserBundle thing left for the next commit).
SonataUserBundle : The "easy-extended" ApplicationSonataUserBundle (in src/).

./console sonata:easy-extends:generate --dest=src/ SonataUserBundle

See http://sonata-project.org/bundles/user/master/doc/reference/installation.html#extending-the-bundle