don't enable fkey checks on _internal database

[cldellow]

dux enables foreign key constraint checking by default on every connection, as part of a prepare_connection hook.

This is a little dangerous -- since fkeys aren't checked by default, some databases may violate them. Still, let's see how far we can get with this as a default.

One issue currently is simonw/datasette#2032 - let's exempt _internal from fkey checking