swarm: move grafana's package to deploy, beside the rest of its host knobs

Same rule as the matrix pair: which build a service runs is a decision of
the host that runs it, so `package` follows `enable` out of the namespace
every hive must agree on byte for byte.

Grafana already had a `deploy.grafana` block holding the datasources, the
plugin list and the socket directory, so this is the package joining
options it belongs with rather than a new namespace.

Prose: the swarm-side header listed "its package" among what stays; the
deploy-side header listed what the host decides without it. Both edited,
which is the fix — a substitution on the first alone would have left the
second silently incomplete.

`swarm-grafana.nix:219` was on the sweep list and is NOT edited: it says
what stays above is what the service is to every hive, and that stays
true once the package leaves. The stale half was the sentence after it.

Refs #3772.

nix/module-eval.nix

@@ -160,6 +160,7 @@ let
     swarm.grafana.datasourceUrl = "http://127.0.0.1:19999";
     swarm.grafana.logsDatasourceUrl = "http://127.0.0.1:19998";
     swarm.grafana.plugins = [ ];
+    swarm.grafana.package = pkgs.emptyDirectory;
   };
 
   # The first slice to leave options on BOTH sides of the split, so the