Switch reverse_relationship_info() to the relcond resolver

Switch reverse_relationship_info() to the relcond resolver

Prompted by a PR from @mzealey, a code audit showed the entire implementation
to be severely lacking. Switched to proper relationship resolution, with the
added benefit of support for custom conds whenever possible.

As of this commit every single relationship introspection now goes through a
central point: _resolve_relationship_condition(). No more random ... eq 'HASH'
checks all over the place.

There should be zero functional changes as a result (aside from better custom
cond introspection)