lib/modules: add definitionsWithLocations to evaluated options

This attr provides the location of each definition.

This is particularly useful for introspecting options of type
`attrsOf`. E.g., it allows finding the location of a systemd
service definition by parsing
`options.systemd.services.definitionsWithLocations`.

lib/modules.nix

@@ -720,6 +720,7 @@ rec {
         inherit (res.defsFinal') highestPrio;
         definitions = map (def: def.value) res.defsFinal;
         files = map (def: def.file) res.defsFinal;
+        definitionsWithLocations = res.defsFinal;
         inherit (res) isDefined;
         # This allows options to be correctly displayed using `${options.path.to.it}`
         __toString = _: showOption loc;