lib/modules: Fix nonexistant option error

The refactoring in https://github.com/NixOS/nixpkgs/commit/fd75dc876586bde8cdb683a6952a41132e8db166
introduced a mistake in the error message that doesn't show the full
context anymore. E.g. with this module:

  options.foo.bar = lib.mkOption {
    type = lib.types.submodule {
      baz = 10;
    };
    default = {};
  };

You'd get the error

  The option `baz' defined in `/home/infinisil/src/nixpkgs/config.nix' does not exist.

instead of the previous

  The option `foo.bar.baz' defined in `/home/infinisil/src/nixpkgs/config.nix' does not exist.

This commit undoes this regression

lib/modules.nix

@@ -115,8 +115,8 @@ rec {
 
       checkUnmatched =
         if config._module.check && config._module.freeformType == null && merged.unmatchedDefns != [] then
-          let inherit (head merged.unmatchedDefns) file prefix;
-          in throw "The option `${showOption prefix}' defined in `${file}' does not exist."
+          let firstDef = head merged.unmatchedDefns;
+          in throw "The option `${showOption (prefix ++ firstDef.prefix)}' defined in `${firstDef.file}' does not exist."
         else null;
 
       result = builtins.seq checkUnmatched {