Motivation for this change

This makes it easier to write configuration systems that can be
reused as submodules, by writing the configuration system as a
type, exposing the type (e.g. in a flake lib attribute), and
using the type's evalModules function in the configuration
system's own wiring.

The alternative, the status quo, is for the configuration system to duplicate its invocation logic when providing both a type and an evalModules-based function. This is error prone. Here's a best case example of the status quo:

let
  type = lib.types.submoduleWith {
    modules = myBuiltInModules;
    inherit specialArgs;
  };
  # Avoidable boilerplate:
  evalConfig = configuration: lib.evalModules {
    modules = [ configuration ] ++ myBuiltInModules;
    inherit specialArgs;
  };
  # With this pr
  evalConfig = configuration: type.evalModules { modules = [ configuration ]; };

Also note that because modules and specialArgs are now referenced only once, we don't need to rely on let bindings as much.

It also turns out the the internal evalTypeModules is a decent abstraction. Wasn't expecting that :)

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 via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 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.