nixos/postgresql: improve the assertions for equality of DB user and DB name

  config = mkIf cfg.enable {

    assertions = [

      {

        assertion = all

          ({ name, ensureDBOwnership, ... }: ensureDBOwnership -> builtins.elem name cfg.ensureDatabases)

          cfg.ensureUsers;

    assertions = map ({ name, ensureDBOwnership, ... }: {

      assertion = ensureDBOwnership -> builtins.elem name cfg.ensureDatabases;

      message = ''

        For each database user defined with `services.postgresql.ensureUsers` and

        `ensureDBOwnership = true;`, a database with the same name must be defined

        in `services.postgresql.ensureDatabases`.

        '';

      }

    ];

        Offender: ${name} has not been found among databases.

      '';

    }) cfg.ensureUsers;

    # `ensurePermissions` is now deprecated, let's avoid it.

    warnings = lib.optional (any ({ ensurePermissions, ... }: ensurePermissions != {}) cfg.ensureUsers) "

      `services.postgresql.*.ensurePermissions` is used in your expressions,