Replace defaultBla.$system with bla.$system.default

[edolstra]

This also simplifies some InstallableFlake logic and fixes nix bundle parsing its installable twice.

Fixes #5532.

TODO

• make nix flake check warn about the old defaultX outputs.
• update documentation
• release note (bonus for examples)

[Pacman99]

How come this doesn't change nixosModule to nixosModules.default? Thats similar enough to overlay since their both not system-spaced and nix functions, but only overlay was changed.

[rehno-lindeque]

I have the same question. I think a nice pattern for many people would be:

nixosModules = rec {
  default = {
    imports = [
      moduleA
      moduleB
      moduleC
    ];
  };
  moduleA = import ./nixos-modules/module-a;
  moduleB = import ./nixos-modules/module-b;
  moduleC = import ./nixos-modules/module-c;
}