pg-introspection doesn't exclude extension table constraints
Using pgsodium, without doing anything to the schema the following error appears when starting graphile.

```
getAttributes() called on constraint 'key_parent_key_fkey' (type = 'f'), but we could not find the constraint's table (oid = '19324') in the introspection results; returning empty array
getForeignAttributes() called on constraint 'key_parent_key_fkey' (type = 'f'), but we could not find the constraint's foreign table (oid = '19324') in the introspection results; returning empty array
```

The `key_parent_key_fkey` constraint is on the pgsodium.key table and points back to itself on pgsodium.key.id

Related discord chat: https://discord.com/channels/489127045289476126/1151774296360894474