lib/modules: fix error-message when declaring an option inside `config'
lib/modules: fix error-message when declaring an option inside `config'

The message I originally implemented here was to catch a mixup of
`config' and `options' in a `types.submodule'[1]. However it looks
rather weird for a wrongly declared top-level option.

So I decided to throw

    error: The option `foo' does not exist. Definition values:
           - In `<unknown-file>':
               {
                 bar = {
                   _type = "option";
                   type = {
                     _type = "option-type";
               ...

           It seems as you're trying to declare an option by placing it into `config' rather than `options'!

for an expression like

    with import ./lib;

    evalModules {
      modules = [
        {
          foo.bar = mkOption {
            type = types.str;
          };
        }
      ];
    }

[1]  fa30c9abed61f30218a211842204705986d486f9

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