lib.evalModules: init transformImport, allow users to preprocess imports

[hsjobeki]

Description of changes

Fyi: i am not sure about the name. Feel free to suggest other ones.

• transformImport
• prepareImport
• processImport
• importHandler

Fixes: #196584

Add transformImport to evalModules.

This allows users to:

• Check imports.
• Transform imports.
• Use imports not supported natively, by downcasting/transforming their custom modules
• Improve ergonomics as shown in the linked issue.

Also note:

How NOT to use the preprocessing:

  eval = lib.evalModules {
    transformImport = module: if eval.config.foo == 42 then module else {};

This causes infinite recursion. Imports cannot depend on config results.

Other considerations

• This adds another attribute to evalModules. (Performance impact should be minimal, as this is only a single pointer)
• Adds a little complexity.
• When using this for custom Modules, will always require their specific transform, which makes them less portable.

Implementation Notes:

I needed to add more arguments to collectModules. I didn't know how to pass the options to collectModules otherwise.

Maybe a better approach would be to preserve compatibility of collectModules outside of evalModules / nixpkgs. I dont know if anyone uses it.

collectModules = collectModulesWith { };

collectModulesWith = {
  transformImport ? ...
}: ...

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• 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/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 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
• Fits CONTRIBUTING.md.

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