Instead of reading from the 'secrets' directory all over the place,
consolidate all secrets-handling inside the same module.
This means that finally, the 'acme' service does not need to come read
right into this repository, however this leads to a potentially unsecure
setup (because I am storing passwords in the Nix store)... I have
decided not to care about this relatively minor issue, but I could
revisit it by using `sops-nix` in the future.
