Combine index_info and find_secondary_indexes into a single routine that

Combine index_info and find_secondary_indexes into a single routine that
returns a list of RelOptInfos, eliminating the need for static state
in index_info.  That static state was a direct cause of coredumps; if
anything decided to elog(ERROR) partway through an index_info search of
pg_index, the next query would try to close a scan pointer that was
pointing at no-longer-valid memory.  Another example of the reasons to
avoid static state variables...