system.requiredKernelConfig: packages contraints

[teto]

This lets packages define meta.requiredKernelConfig that is added to system.requiredKernelConfig when the package belongs to environment.systemPackages.
This checks if the package can work with the booted kernel.
(hopefully the check could be run against an arbitrary kernel).

The hope is to then add a knob that builds a kernel configuration satisfying the installed package constraints.

Motivation for this change

Long term goal is #41103. The intent with the PR is just to start with a smaller scope and concrete code. This is not the definite proposition but the main idea is visible and I hope to trigger some feedback.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[teto]

so I just wonder if the idea could make it into nixos (disabled by default as it's hard to automatically build a correct kernel config) ? then I can refine the implementation (which is a copy/paste hack right now).
In a nutshell, if you set boot.enforceRequiredConfig = true and add openvswitch to systemPackages, nix adds CONFIG_OPENVSWITCH to the config.

[teto]

while trying to clean it, I found some flaws, the fix is non trivial else I will submit another proposal in a few weeks.