Improve tab completion of IMPORT FOREIGN SCHEMA in psql

Improve tab completion of IMPORT FOREIGN SCHEMA in psql

It is not possible to get a list of foreign schemas as the server is not
known, so this provides instead a list of local schemas, which is more
useful than nothing if using a loopback server or having schema names
matching in the local and remote servers.

Author: Jeff Janes
Reviewed-by: Tom Lane, Michael Paquier
Discussion: https://postgr.es/m/CAMkU=1wr7Roj41q-XiJs=Uyc2xCmHhcGGy7J-peJQK-e+w=ghw@mail.gmail.com