Adjust join_search_one_level's handling of clauseless joins.

For an initial relation that lacks any join clauses (that is, it has to be
cartesian-product-joined to the rest of the query), we considered only
cartesian joins with initial rels appearing later in the initial-relations
list.  This creates an undesirable dependency on FROM-list order.  We would
never fail to find a plan, but perhaps we might not find the best available
plan.  Noted while discussing the logic with Amit Kapila.

Improve the comments a bit in this area, too.

Arguably this is a bug fix, but given the lack of complaints from the
field I'll refrain from back-patching.

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