... i.e. without adding any new strictness

This adds support for warnings in submodules.

Fixes #96006

To recap, #97023 didn't make it because of unexpected infinite recursions. Context:

• Attaching a warning to an expression can create an otherwise unnecessary data dependency, which can lead to infinite mutual recursion when a (typically very necessary) dependency exists in the other direction.
• The warnings option is largely independent of the evaluation of the configuration, because it only reads from it. Nothing except system.build.toplevel reads from warnings. Not cyclical, everyone happy.
• [Reverted] Module-builtin assertions, disabling assertions and submodule assertions #97023 created data dependencies at submodule roots, not just toplevel

While something like #97023 is a very general solution that would solve this mkRenamedOptionModule with ease, we don't need it for this particular problem.
Specifically, in this case, we already have a data dependency where the new option has a definition based on the old option. So, we can attach the
warning to the mkIf condition of that definition and make it trigger when
the old option is about to be read anyway.

As a side effect of this change, these particular warnings don't appear in the warnings option anymore. Maybe someone used those for testing, but mkRenamed* usages aren't really worth testing, so this is a small price to pay for support for renames in submodules.

TODO

• more tests (this function did not have any tests in lib/tests/modules.sh)

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/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 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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