Fix adding a foreign key in non-default schema

And make the search path consistent for general and grouping foreign keys.

Previously, ADD GROUPING FOREIGN KEY would look up an unqualified referenced table in the same schema as the table being altered. Adding a non-grouping would look up in the default schema. As we don't have elaborate search paths, always using the default schema seems the clearest and most consistent.

This is a behavior change but with a pretty small surface area, I think. fdbsqldump always fully qualifies and I'm guessing manually created cross-schema GFKs are otherwise uncommon.