Nixery set

This folder exports a special import of the depot Nix structure that is compatible with Nixery, by extending nixpkgs with a tvl attribute containing the depot.

This is required because Nixery expects its package set to look like nixpkgs at the top-level.

In the future we might want to patch Nixery to not require this (e.g. make it possible to pass third_party.nixpkgs as a key at which to find the nixpkgs structure).