refactor(ops): Split //ops/nixos into different locations

Splits //ops/nixos into:

* //ops/nixos.nix - utility functions for building systems
* //ops/machines - shared machine definitions (read by readTree)
* //ops/modules - shared NixOS modules (skipped by readTree)

This simplifies working with the configuration fixpoint in whitby, and
is overall a bit more in line with how NixOS systems in user folders
currently work.

Change-Id: I1322ec5cc76c0207c099c05d44828a3df0b3ffc1
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2931
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: glittershark <grfn@gws.fyi>

ops/modules/irccat.nix

@@ -0,0 +1,49 @@
+{ depot, config, lib, pkgs, ... }:
+
+let
+  cfg = config.services.depot.irccat;
+  description = "irccat - forward messages to IRC";
+
+  # irccat expects to read its configuration from the *current
+  # directory*, and its configuration contains secrets.
+  #
+  # To make this work we construct the JSON configuration file and
+  # then recursively merge it with an on-disk secret using jq on
+  # service launch.
+  configJson = pkgs.writeText "irccat.json" (builtins.toJSON cfg.config);
+  configMerge = pkgs.writeShellScript "merge-irccat-config" ''
+    if [ ! -f "/etc/secrets/irccat.json" ]; then
+      echo "irccat secrets file is missing"
+      exit 1
+    fi
+
+    # jq's * is the recursive merge operator
+    ${pkgs.jq}/bin/jq -s '.[0] * .[1]' ${configJson} /etc/secrets/irccat.json \
+      > /var/lib/irccat/irccat.json
+  '';
+in {
+  options.services.depot.irccat = {
+    enable = lib.mkEnableOption description;
+
+    config = lib.mkOption {
+      type = lib.types.attrs; # varying value types
+      description = "Configuration structure (unchecked!)";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    systemd.services.irccat = {
+      inherit description;
+      preStart = "${configMerge}";
+      script = "${depot.third_party.irccat}/bin/irccat";
+      wantedBy = [ "multi-user.target" ];
+
+      serviceConfig = {
+        DynamicUser = true;
+        StateDirectory = "irccat";
+        WorkingDirectory = "/var/lib/irccat";
+        Restart = "always";
+      };
+    };
+  };
+}