Better octodns file structure

dns/implementation/nixos-module-assertions.nix

@@ -0,0 +1,17 @@
+# This must only be loaded on actual Nixos, otherwise `assertions`
+# won't be defined when running `evaluateModules`.
+{ config, ... }:
+let cfg = config.satellite.dns;
+in
+{
+  config.assertions =
+    let assertProperToUsage = config:
+      {
+        assertion = (config.to == null) || (config.type == "CNAME");
+        message = ''
+          The option `satellite.dns.records[*].to` can only be used with `CNAME` records.
+          This was not the case for ${config.type} record at ${config.at}.${config.zone}.
+        '';
+      };
+    in builtins.map assertProperToUsage cfg.records;
+}