Comment 0 for bug 1563101

Remove dependencies where backend code references code in the core. The reasoning being that the core should know about the backends, but the backends should not know anything about the core (separation of concerns). And part of the risk here is a potential for circular dependencies.

For example, identity backends imports identity:
https://github.com/openstack/keystone/blob/master/keystone/identity/backends/sql.py#L24

Backend code could then reference code in the core, as well as other modules inside identity; thus, creating a circular dependency.