Automatically determine boot libs that must be overridden

[kirelagin]

When there is a boot library in extra-deps, we need to find all other boot libraries that we have in our transitive dependencies which depend on this one and override them as well.

[kirelagin]

Here is a snippet that collects names and expected versions of boot packages.

  sedProgram =
    ''/^name:/N;s/name: (.*)\nversion: (.*)$/"\1" = "\2";/p;'';
  getBootPkgsDrv = ghc:
    pkgs.runCommand "nixage-boot-pkgs" {
      nativeBuildInputs = [ ghc ];
    } ''echo "{" >> "$out"
        ghc-pkg                  \
          --no-user-package-conf \
          dump                   \
        | sed -rne               \
            '${sedProgram}'      \
        >> "$out"
        echo "}" >> "$out"
    '';
  getBootPkgs = ghc: import (getBootPkgsDrv ghc);

  canOverride = n: v:
    !(elem n [
      "ghc"
      "ghc-prim"
      "rts"
      "base"
      "integer-gmp"
    ]);
  getOverridableBootPkgs = ghc: filterAttrs canOverride (getBootPkgs ghc);

[kirelagin]

A straightforward implementation of this could be to simply see if our overrides override any of the boot packages and if this is the case, override them all with callHackage (of course before applying overrides form the project spec!).
This will potentially require to rebuild more boot packages than required, but whatever, there are not that many of them.