lib/options: Add showOptionWithDefLocs

lib/options.nix

@@ -7,6 +7,7 @@ let
     collect
     concatLists
     concatMap
+    concatMapStringsSep
     elemAt
     filter
     foldl'
@@ -340,6 +341,11 @@ rec {
     in "\n- In `${def.file}'${result}"
   ) defs;
 
+  showOptionWithDefLocs = opt: ''
+      ${showOption opt.loc}, with values defined in:
+      ${concatMapStringsSep "\n" (defFile: "  - ${defFile}") opt.files}
+    '';
+
   unknownModule = "<unknown-file>";
 
 }