Add extraDebug, lint

[roberth]

      Diagnostic messages that a flake author may or may not care about.

      For example, a module might detect that it's used in a weird way, but
      not be sure whether that's a mistake or not. Emitting a warning would
      be too much, but with this option, the author can still find the
      detected problem, by enabling [`debug`](#opt-debug) and querying
      the `debug.lint.messages` flake attribute in `nix repl`.

      This feature is not gated by an enable option, as performance does not
      suffer from an unevaluated option.

      There's also no option to upgrade to warnings, because that would make
      evaluation dependent on rather many options, even if the caller only
      needs one specific unrelated thing from the flake.

      A more complex interface could attach the warnings to specific flake
      attribute paths, but that's not implemented for now.