We've grown a lot of modules in this repo and it's hard to see the wood
for the trees when you look in the `src` directory. This commit attempts
to neaten everything up a bit by scoping some of them under collective
parent modules:
* `auth_db`, `delivery_problems` and `message_data` have moved to `db`,
  and the old `db` has moved to `db/core`.
* `healthcheck` and `send` have moved to `api`.
* `app_errors`, `duration`, `email_address` and `validate` have moved to
  `types`, and `app_errors` is now just `error`.
* `serialize` has moved to `settings` as it was only referenced there.
Hopefully the end result makes it easier to zero in on modules of
interest when working in the repo.