psql: show cloned triggers in partitions

psql: show cloned triggers in partitions

In a partition, row triggers that had been cloned from their parent
partitioned table would not be listed at all in psql's \d, which could
surprise users, per insistent complaint from Ashutosh Bapat (though his
aim was elsewhere).  The simplest possible fix, suggested by Peter
Eisentraut, seems to be to list triggers marked as internal if they have
a row in pg_depend that points to some other trigger.

Author: Álvaro Herrera
Discussion: https://postgr.es/m/20180618165910.p26vhk7dpq65ix54@alvherre.pgsql