hackage2nix: Split configuration, auto disable hydra builds

We split configuration-hackage2nix.yaml into multiple files.  We bump
cabal2nix-unstable to get support for multiple config files in
hackage2nix.

* The file main.yaml is only supposed to be edited by humans.
* The file stackage.yaml is only supposed to be updated by the
  update-stackage.sh
* The file broken.yaml can be edited by humans, but probably future
  helpers will want to insert broken packages into this file based on
  hydra reports.
* The file transitive-broken.yaml is newly introduced to be generated
  by regenerate-transitive-broken-packages.sh

regenerate-transitive-broken-packages.sh makes a nix query (in
transitive-broken-packages.nix) which evaluates all haskellPackages
once with and once without "allowBroken" this way it get's a list of
packages which are broken by some transitive dependency, but does not
disable packages which have eval errors not caused by a broken package.

maintainers/scripts/haskell/transitive-broken-packages.nix

@@ -0,0 +1,21 @@
+let
+  nixpkgs = import ../../..;
+  inherit (nixpkgs {}) pkgs lib;
+  getEvaluating = x:
+    builtins.attrNames (
+      lib.filterAttrs (
+        _: v: (builtins.tryEval (v.outPath or null)).success && lib.isDerivation v && !v.meta.broken
+      ) x
+    );
+  brokenDeps = lib.subtractLists
+    (getEvaluating pkgs.haskellPackages)
+    (getEvaluating (nixpkgs { config.allowBroken = true; }).haskellPackages);
+in
+''
+  # This file is automatically generated by
+  # maintainers/scripts/haskell/regenerate-transitive-broken-packages.sh
+  # It is supposed to list all haskellPackages that cannot evaluate because they
+  # depend on a dependency marked as broken.
+  dont-distribute-packages:
+  ${lib.concatMapStringsSep "\n" (x: "  - ${x}") brokenDeps}
+''