Feature: --granular-output flag

[mrcjkb]

This adds a --granular-output flag that allows to split a package into multiple derivations (where executable/test/benchmark targets depend on the libraries). The behaviour without the flag is unchanged.

See also: NixOS/nixpkgs#315129

Example output:

cabal2nix cabal2nix/test/golden-test-cases/ttrie.cabal --granular-output

{ mkDerivation, async, atomic-primops, base, bifunctors, containers
, criterion-plus, deepseq, hashable, lib, mwc-random, primitive
, QuickCheck, stm, stm-containers, stm-stats, test-framework
, test-framework-quickcheck2, text, transformers
, unordered-containers, vector
}:
let
  ttrie = mkDerivation {
            pname = "ttrie";
            version = "0.1.2.1";
            sha256 = "deadbeef";
            libraryHaskellDepends = [
              atomic-primops base hashable primitive stm
            ];
            doCheck = false;
            buildTarget = "ttrie";
            homepage = "http://github.com/mcschroeder/ttrie";
            description = "Contention-free STM hash map";
            license = lib.licenses.mit;
          };
in {
  libraries = {inherit ttrie;};
  exes = { };
  testExes = {
     map-properties = mkDerivation {
                        pname = "map-properties";
                        version = "0.1.2.1";
                        sha256 = "deadbeef";
                        isLibrary = false;
                        isExecutable = true;
                        testHaskellDepends = [
                          base containers hashable QuickCheck stm test-framework
                          test-framework-quickcheck2 ttrie
                        ];
                        buildTarget = "map-properties";
                        homepage = "http://github.com/mcschroeder/ttrie";
                        description = "Contention-free STM hash map";
                        license = lib.licenses.mit;
                      };
  };
  benchExes = {
      bench = mkDerivation {
                pname = "bench";
                version = "0.1.2.1";
                sha256 = "deadbeef";
                isLibrary = false;
                isExecutable = true;
                benchmarkHaskellDepends = [
                  async base bifunctors containers criterion-plus deepseq mwc-random
                  primitive stm stm-containers stm-stats text transformers ttrie
                  unordered-containers vector
                ];
                doBenchmark = true;
                buildTarget = "bench";
                homepage = "http://github.com/mcschroeder/ttrie";
                description = "Contention-free STM hash map";
                license = lib.licenses.mit;
              };
   };
}

I've hit a dead end with this, which is why I'm opening this as a draft PR to find out if it's worth pursuing this idea further:

Even if an executable derivation has the library/libraries in its *HaskellDepends, cabal will prioritise building the library from source, essentially rendering this feature useless.
I've tried getting this to work with incremental builds (setting doInstallIntermediates and enableSeparateIntermediatesOutput in the libraries and passing them to the exes via previousIntermediates), but this doesn't seem to help.

Maybe someone here has an idea how to solve that problem?

Important

In the current state, the *Exes targets don't work if the source is hackage.