lib.composeExtensions: unify the api to use overlays

[hsjobeki]

Description of changes

This PR tries to unify the API to use and apply overlays.

composeExtensions a b is equivalent to:
composeManyExtensions [a b]

But composeManyExtensions is more generic since it can be extended composeManyExtensions [a b ...]

Due to lacking documentation and inherent API for overlays some package set maintainers re-implemented composeManyExtensions as:

lib.foldr  (composeExtensions [ ... ] (final: prev: {});

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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[hsjobeki]

example for an identical re-implementation of composeManyExtensions

-> lib.foldr lib.composeExtensions (self: super: { }) ([ ...])

Is literally the same as composeManyExtensions (minus the lambda reduction).

composeManyExtensions = overlays: lib.foldr (x: y: composeExtensions x y) (final: prev: {}) overlays;

[hsjobeki]

cc @roberth This is what i proposed in #325479.
If we have to maintain duplicate documentation, this is because we have two api functions that do the same thing basically. (or one is a strict superset of the other)