Further tradeoffs on relationship sanity checking (augments 9e7525a2)

Further tradeoffs on relationship sanity checking (augments 9e7525a2)

It turns out what we did earlier is not enough - what is truly needed is
shutting off *all* checks on non-belongs_to rels. Otherwise industrious devs
will inevitable tie themselves into a circual loading order knot.

The problem (as in 9e7525a2) is not so much that we need to bend over
backwards to support circular loads, but that these problems could very
well be masked by stable hash ordering of perl < 5.18. In other words this
commit serves to prevent perfectly working code from suffering intermittent
and hard-to-debug issues when someone upgrades their perl in 2018.