ALTER FOREIGN DATA WRAPPER can drop dependency on handler

CREATE FUNCTION test_fdw_handler()
RETURNS fdw_handler
AS :'regresslib', 'test_fdw_handler'
LANGUAGE C;

CREATE FOREIGN DATA WRAPPER test_fdw HANDLER test_fdw_handler;

-- correctly raises dependency ERROR
DROP FUNCTION test_fdw_handler;

ALTER FOREIGN DATA WRAPPER test_fdw VALIDATOR postgresql_fdw_validator;

-- fails to raise dependency error ERROR
DROP FUNCTION test_fdw_handler;

Patch attached.

Note: I found this while investigating a related issue for the FDW
connection function, with patch here:

https://www.postgresql.org/message-id/fd49b44dc65da8e71ab20c1cf1ec7e65921c20f5.camel@j-davis.com

I am keeping this patch separate for a clean backport.

Regards,
Jeff Davis