cli: use prometheus compatible structure

When invoked with `-s rules|alerts` we now include the `groups:` prefix
so that the output can directly be piped into Prometheus rule files

cmd/pop/eval.go

@@ -29,9 +29,9 @@ func mixCmd() *cli.Command {
 		var out interface{} = mix
 		switch *system {
 		case "alerts":
-			out = mix.PrometheusAlerts
+			out = map[string]interface{}{"groups": mix.PrometheusAlerts}
 		case "rules":
-			out = mix.PrometheusRules
+			out = map[string]interface{}{"groups": mix.PrometheusRules}
 		case "grafana":
 			out = mix.GrafanaDashboards
 		}

pkg/pop/pop.go

@@ -65,6 +65,10 @@ func (p Pop) Mixin() (*Mixin, error) {
 		return nil, mixed.Err()
 	}
 
+	if err := mixed.Validate(cue.Concrete(true)); err != nil {
+		return nil, err
+	}
+
 	mixin := mixer.LookupPath(cue.ParsePath("mixin"))
 
 	m := Mixin{