Declare preferences with a type and a predefined key, and lay mesh and node config between default and node (hq issue 630)

A preference may fall back to one reviewed mesh-wide or node-wide value instead of a module layer for the whole mesh; like a default, config fills ${setting:} only.

internal/catalogue/setting_takes.go

@@ -43,7 +43,7 @@ func onlyTaken(layers []Layer, taken map[string]bool) []Layer {
 				values[key] = value
 			}
 		}
-		kept = append(kept, Layer{From: layer.From, Values: values, Default: layer.Default})
+		kept = append(kept, Layer{From: layer.From, Values: values, Default: layer.Default, Config: layer.Config})
 	}
 	return kept
 }
@@ -83,7 +83,7 @@ func TakenSettingsProblems(m Manifest) []string {
 // Said only when none of the module's mergeable files names its keys: when one does, §2 already
 // names every file that does not, and a file is not reported twice.
 func WrittenSettingsProblems(m Manifest) []string {
-	if len(m.Settings) == 0 {
+	if len(PreferencesOf(m)) == 0 {
 		return nil
 	}
 	var silent []string