Add an extra RV to the relationship resolver

Add an extra RV to the relationship resolver

A certain spot in the codebase check whether a relationship is "simple".
This additional flag allows to consider coderef conditions as well, instead
of simply punting with "not a HASH? - no can do"

See next commit for the actual switchover

While at it fix a subtle bug introduced in b5ce6748 - originally the helper
is_literal_value recognized -ident as a valid literal. Later on during the
migration into SQLA this logic was lost (I do not exactly recall the details),
yet the DBIC side was never adjusted. All callsites were audited to make sure
nothing else was missed.