Change get_constraint_index() to use pg_constraint.conindid

Change get_constraint_index() to use pg_constraint.conindid

It was still using a scan of pg_depend instead of using the conindid
column that has been added since.

Since it is now just a catalog lookup wrapper and not related to
pg_depend, move from pg_depend.c to lsyscache.c.

Reviewed-by: Matthias van de Meent 
Reviewed-by: Tom Lane 
Reviewed-by: Michael Paquier 
Discussion: https://www.postgresql.org/message-id/flat/4688d55c-9a2e-9a5a-d166-5f24fe0bf8db%40enterprisedb.com