`gather.initialState` needs access to `EXPORTABLE` and `sql`
E.g. look at PgLtreePlugin - it had to import from graphile-build and pg-sql2 directly!

`info.EXPORTABLE` on helpers exists, but there's no `sql` equivalent. Helpers should be hookable, and we should have a helpers hook so that plugins can add stuff to helpers (like adding to build).

Alternatively, `gather()` should have access to `Build`... but if so, we need to rethink `schema.hooks.build` since it needs to exist before `gather`.