A served value or a contribution's may be the operator's: ${setting:…} fills there too, refused by name when unset

Issue 173's rule needs it: a mail provider serves its domain and an identity provider its issuer,
and neither is the definition's to state. Declared as ${setting:<key>}, filled from the layers after
the overrides; a key so asked for is not stray.

internal/catalogue/setting_into.go

@@ -91,19 +91,40 @@ func orNoSettings(layers []Layer) string {
 	return "; set today: " + strings.Join(keys, ", ")
 }
 
-// settingKeysUsedBy is every key a module's files ask for, so a setting that lands in one is not
-// called stray.
+// settingKeysUsedBy is every key a module's files, contributions and served facts ask for, so a
+// setting that lands in one is not called stray.
 func settingKeysUsedBy(m Manifest) map[string]bool {
 	used := map[string]bool{}
+	note := func(s string) {
+		for _, k := range settingsUsed(s) {
+			used[k] = true
+		}
+	}
 	for _, r := range m.Resources {
 		if fmt.Sprint(r["type"]) != "file" {
 			continue
 		}
 		if content, ok := r["content"].(string); ok {
-			for _, k := range settingsUsed(content) {
-				used[k] = true
+			note(content)
+		}
+	}
+	inValues := func(values map[string]any) {
+		for _, v := range values {
+			if s, ok := v.(string); ok {
+				note(s)
 			}
 		}
 	}
+	for _, values := range m.Contributes {
+		inValues(values)
+	}
+	for _, locals := range m.ContributesMany {
+		for _, values := range locals {
+			inValues(values)
+		}
+	}
+	for _, values := range m.Serves {
+		inValues(values)
+	}
 	return used
 }