Ensure the custom rel cond resolver does not trigger forgotten compat shim

During the rush to get custom rels out the door (this is why rushing
fucking sucks), a697fa31 introduced a shortsighted workaround into
::SQLMaker::_from_chunk_to_sql(). This code slipped consequent review
and made its way into the codebase... 4 FUCKING YEARS AGO!!! >:(

Since it is not known how much stuff relies on the insanity being there
(moreover we have tests that rely on it) leave things as is for the time
being. The only change is  making the cond resolver *completely* oblivious to
the "single-element hash" workaround (albeit via a silly hack).

In the process exposed that ora-joins module is entirely incapable
of understanding non-equality conds... fml

See next commits for added warnings, etc.
Split DBIC from SQLMaker test (deprecated in next commit)
Put in place deprecation forgotten for several years

More info and rationale in the commit msg of c200d949