nixpkgs: rebase module on latest NixOS

This is based on the current NixOS `nixpkgs` module, adjusted for the
nix-darwin context and without adding the options due for deprecation
in NixOS.

This gives us the ability to set the package set modularly through
`nixpkgs.pkgs` and builds up infrastructure for handling user-specified
Nixpkgs instantiations more robustly.

The cross-compilation options are currently not very useful due to
even Darwin->Darwin cross-compilation not being wholly functional
yet, but it looks feasible to build an `aarch64-darwin` system from
`x86_64-darwin` with some patching and it should be possible to make
cross-compilation more widely supported after the Darwin SDK situation
in Nixpkgs improves.

One casualty is the error for setting `nixpkgs.*` options when
overriding the package set. That could be ported over to this new
scheme, but it'd increase divergence with the NixOS module and reduce
cross-compatibility of configurations, so I lean towards adding it
upstream to NixOS if anything. (But if people want to keep it I can add
it back.)

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