Assumption: The stack build does not depend on which nixpkgs version it uses, it only needs a matching ghc version

~/.cache/all-hies/ layout:

- history-nixpkgs-unstable
- nixpkgs
- haskell-ide-engine
- per-nixpkgs
  - <nixpkgsrev-ghcVersions>: File with a list of ghc versions this nixpkgs rev has
- per-haskell-ide-engine
  - <hie-rev>
    - <ghcVersion-stack2nix>.nix: File generated by stack2nix for hie-rev and ghcVersion

./generated/ layout:
- nixpkgsForGhc
  - <ghcVersion>: File containing the nixpkgsrev to use for that ghc
- stable
  - <ghcVersion>.nix: File copied from the cache
  - revision: File with the git revision of current stable hie
- unstable
  - <ghcVersion>.nix: File copied from the cache
  - revision: File with the git revision of current unstable hie

Flags:
- Whether master should be regenerated
- Whether to build the generated versions with Nix

Process:
- Update/initialize hie repo
- Regenerate ./generated/stable by
  - Removing ./generated/stable/*
  - Getting the latest tag (~=stable)
  - Write stable tag to ./generated/stable/revision
  - Determining the stable hie sha1 hash from the tag
  - Checking out that hie revision
  - For each stack file for a ghc version
    - If $CACHE/per-hie/<rev>/<ghcVersion>.nix is present
      - copy it to ./generated/stable/<ghcVersion>.nix
    - else generate that file by
      - Find a suitable nixpkgs revision for that ghc version
        - If ./generated/nixpkgsForGhc/<ghcVersion> present, use that version
        - else
          - fetch channel history (or use cached version if <15 minutes ago)
          - for each channel nixpkgs revision (from newest to oldest)
            - Get ghc versions
              - If $CACHE/per-nixpkgs/<revision> exists use that
              - Otherwise
                - Check out that nixpkgs version (and maybe update/initialize the repo first)
                - Run nix evaluation to determine available compilers
                - Save result in $CACHE/per-nixpkgs/<revision>
            - If it has the ghc version we need
              - Save that revision in ./generated/nixpkgsForGhc/<ghcVersion>
              - revision found, exit channel history loop
            - Otherwise continue
          - If no fitting ghc version was found, log error
      - Checking out that nixpkgs revision
      - Invoke stack2nix to generate $CACHE/per-hie/<rev>/<ghcVersion>.nix
      - If nix build flag is enabled, build it
      - Copy $CACHE/per-hie/<rev>/<ghcVersion>.nix to ./generated/stable/<ghcVersion>.nix
- If master flag is enabled, regenerate ./generated/unstable by
  - (same process as above, only difference is first step)