Um, corehq IS dependent on the billing submodule.

./corehq/apps/hqwebapp/models.py:from hqbilling.dispatcher import BillingInterfaceDispatcher
./corehq/apps/domain/views.py:        from hqbilling.forms import DomainBillingInfoForm
./corehq/apps/sms/util.py:        from hqbilling.tasks import bill_client_for_sms
./corehq/apps/sms/util.py:        from hqbilling.models import API_TO_BILLABLE

Was there a circular import problem?