engine: Don't force validation for hidden resources

I think this is what I want in most scenarios, is there a reason to do
otherwise? This is because we may wish to export incomplete resources,
where the remaining necessary fields for validation happens on collect.

engine/metaparams.go

@@ -154,7 +154,8 @@ type MetaParams struct {
 	// named resources. It can even be used as part of an edge or via a
 	// send/recv receiver. It can NOT be a sending vertex. These properties
 	// differentiate the use of this instead of simply wrapping a resource
-	// in an "if" statement.
+	// in an "if" statement. If it is hidden, then it does not need to pass
+	// the resource Validate method step.
 	Hidden bool `yaml:"hidden"`
 
 	// Export is a list of hostnames (and/or the special "*" entry) which if

engine/resources.go

@@ -312,6 +312,12 @@ func Validate(res Res) error {
 		return fmt.Errorf("the Res name starts with a $")
 	}
 
+	// Don't need to validate normally if hidden.
+	// XXX: Check if it's also Exported too? len(res.MetaParams.Export) > 0
+	if res.MetaParams().Hidden {
+		return nil
+	}
+
 	return res.Validate()
 }