lib/modules: prepareImport, nixos: import the flake default module

[roberth]

Description of changes

Also motivated by

• Add option for defining flake-parts modules for downstream flakes. hercules-ci/flake-parts#61

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/)
• 22.11 Release Notes (or backporting 22.05 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.

[shlevy]

As I said on #196584, I don't think this should go here. It's a "policy vs mechanism" thing, we can just use functions like hercules-ci/flake-parts@7450589 at the call-site.

[roberth]

Closing in favor of #197547

This solution was too powerful. It allowed the preprocessing function to be abused, making modules harder to understand, because a module system application could add crazy logic to the imports preprocessing function.