Motivation for this change

An easy-to-make mistake when declaring e.g. a submodule is the accidental
confusion of options and config:

types.submodule {
  config = {
    foo = mkOption { /* ... */ };
  };
}

However the error-message

  The option `[definition 1-entry 1].foo' defined in `<expr.nix>' does not exist.

is fairly unhelpful because it seems as the options are declared at the
first sight. In fact, it took a colleague and me a while to track down such
a mistake a few days ago and we both agreed that this should be somehow caught
to save the time we spent debugging the module in question.

At first I decided to catch this error in the submodules-type directly
by checking whether options is undeclared, however this becomes fairly
complicated as soon as a submodule-declaration e.g. depends on existing
config-values which would've lead to some ugly builtins.tryExec-heuristic.

This patch now simply checks if the option's prefix has any options
defined if a point in evaluation is reached where it's clear that the
option in question doesn't exist. This means that this patch doesn't
change the logic of the module system, it only provides a more detailed
error in certain cases:

  The option `[definition 1-entry 1].foo' defined in `<expr.nix>' does not exist.

  However it seems as there are no options defined in [definition 1-entry 1]. Are you sure you've
  declared your options properly? This happens if you e.g. declared your options in `types.submodule'
  under `config' rather than `options'.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.