lib/modules: Add mkTransform

[roberth]

Motivation for this change

Allow configs to transform defaults by means of a function, allow "mkForce" to transform regular configs.

Draft because of remaining questions:

• Is this something we want or a symptom of bad module design?

• Should mkTransform produce an override or its own type? Is it ok to peek inside a mkForce value to look for a transform object, or would that be too strict?

• What is the performance impact?

TODO (assuming boring answers to the questions):

• add mkForcePriority etc
• add docs
• fix systemd's use of what should be module system internals

cc @infinisil

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]

Closing, because while this is a better solution than #157070, it moves the module system in the wrong direction. See #157070 (comment)

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/scalpel-minimally-invasive-secrets-provisioning-to-generated-config-files/19845/2

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/natural-deduction-rules-for-the-nixos-module-merge/26513/6