let ALTER COLUMN SET DATA TYPE cope with trigger dependency

hi.

demo:
CREATE TABLE main_table (a int, b int);
CREATE FUNCTION trigger_func() RETURNS trigger LANGUAGE plpgsql AS '
BEGIN
    RAISE NOTICE ''trigger_func(%) called: action = %, when = %, level = %'',
                  TG_ARGV[0], TG_OP, TG_WHEN, TG_LEVEL;
    RETURN NULL;
END;';
CREATE TRIGGER modified_a BEFORE UPDATE OF a ON main_table
FOR EACH ROW WHEN (OLD.a <> NEW.a) EXECUTE PROCEDURE trigger_func('modified_a');

ALTER TABLE main_table ALTER COLUMN a SET DATA TYPE INT8;
ERROR:  cannot alter type of a column used in a trigger definition
DETAIL:  trigger modified_a on table main_table depends on column "a"

With the attached patch, the previous error will no longer occur.
Foreign key–related internal triggers are not directly dependent on the relation
itself; instead, they depend directly on the constraint.
Therefore, we don't need to worry about internal triggers in this context.

v1-0001: "refactor CreateTrigger and CreateTriggerFiringOn".
used also in https://postgr.es/m/CACJufxHJAr2FjbeB6ghg_-N5dxX5JVnjKSLOUxOyt4TeaAWQkg@mail.gmail.com
v1-0002, the actual implementation.