lib.types.submoduleWith: Avoid _key collisions after extendModules (issue #168767)

[roberth]

This fixes module key collisions induced by extendModules
when using anonymous modules.

TODO

• clean up
• consider more elegant solution, like calling unifyModuleSyntax in evalModules instead of in submoduleWith
  • too big a change to evalModules semantics

Description of changes

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[roberth]

I didn't recognize the problem at the time.

The assertion looks for attrNames config.variants.foo.variants.bar.attrs.

    attrs.a = 1;
    variants.foo.attrs.b = 1;
    variants.bar.attrs.y = 1;
    variants.foo.variants.bar.attrs.z = 1;
    variants.foo.variants.foo.attrs.c = 3;

b occurs in the foo variant, so it should also occur in the foo+bar variant.

[roberth]

Ping @infinisil