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.