[MBQL lib] Match implicitly joined refs with multiple paths (#49446)

The ref (usually) contains enough information to unambiguously find the
matching implictly-joined column, even if there are several FKs that
could join the target table.

The remaining ambiguous case is if the *same* FK column exists
multiple times - current MBQL uses raw field IDs for the FK field, so it
can't distinguish between eg. an FK in the main table and the one from a
self-join.