we shouldn't be referencing `riak_kv` in `riak_core`. Besides tests the one place we do this is `hashtree.erl` and that is to support rolling upgrades from previous versions of riak where `hashtree.erl` was in `riak_kv`. It seems like this function should just look for a "registered" `extra_text` callback mod, which `riak_kv_app` or something should register.