fix: returning a validation error in case of wrong configuration.

As of now, while using deck to create a
route-scoped plugin with a `service` field
defined, the sync goes through the first
time but errors out subsequently. Under a
route, we do not want the user to add the
`service` field for a plugin. Thus, we are
checking for this specific scenario and
erroring out, instead of letting the sync
action go through.

For: Kong/deck#1353

pkg/file/builder.go

@@ -1436,6 +1436,10 @@ func (b *stateBuilder) ingestRoute(r FRoute) error {
 	// plugins for the route
 	var plugins []FPlugin
 	for _, p := range r.Plugins {
+		if p.Service != nil && !utils.Empty(p.Service.ID) {
+			return fmt.Errorf("nesting service (%v) under route-scoped plugin (%v) is not allowed", *p.Service.ID, *p.Name)
+		}
+
 		p.Route = utils.GetRouteReference(r.Route)
 		plugins = append(plugins, *p)
 	}