feat(lib): add mkPackagesFor for overlay-friendly package sets

[r-vdp]

Problem

A blueprint flake that wants to expose its packages/ as a nixpkgs overlay
currently has no good option. The obvious

overlays.default = final: _: {
  foo = self.packages.${final.stdenv.hostPlatform.system};
};

doesn't build anything against final — it hands back the set blueprint
already built from inputs.nixpkgs (with whatever nixpkgs.config/overlays
the flake configured). The consumer ends up evaluating a second, independent
nixpkgs instance for those packages. inputs.nixpkgs.follows doesn't help; it
only changes which source gets re-imported, but still imports a second instance.

Concretely, in a NixOS config consuming llm-agents.nix (which uses exactly
this pattern), this accounts for ~2M extra function calls / ~9% of total eval
time per host.

Change

Add a mkPackagesFor output that reloads the packages/ tree against a
caller-supplied pkgs. Packages see the same scope arguments as in
packages.<system> (pkgs, flake, inputs, system, perSystem,
pname); only pkgs is swapped. perSystem.self resolves within the new
scope so intra-set references stay on the caller's nixpkgs.

outputs = inputs:
  let bp = inputs.blueprint { inherit inputs; };
  in bp // {
    overlays.default = final: _prev: {
      foo = bp.mkPackagesFor final;
    };
  };

unfilteredPackages is redefined in terms of mkPackagesFor, so there is a
single loader for the packages/ tree. Existing outputs are unchanged
(verified: identical drv hashes for packages and devShells via
llm-agents.nix as a fixture).

Related: #90, numtide/llm-agents.nix#1172