Configurable auto-wiring

[srid]

Currently haskell-flake implicitly sets packages, devShells and checks. Let's call it "auto wiring".

Unless upstream implements it, we can add an haskellProjects.foo.autoWire bool option (default is true) that users can disable if they want to "wire" the flake outputs themselves, eg.:

{
  haskellProjects.foo = {
    ..
    autoWire = false;
  };
  packages.default = config.haskellProjects.foo.output.packages.mypkg;
  devShells.default = config.haskellProjects.foo.output.devShell;
  # Disabling checks temporarily
  checks = config.haskellProjects.foo.output.checks;
}

[srid]

In the context of #84, here's an alternate API:

{
  flake.haskellFlake.autoWire = true;
}

Note: flake.haskellFlake.autoWire operates only on the haskellProject.default project, and it is enabled by default. Therefore, by default, outputs are auto-wired for the "default" project. But other projects are not injected into the flake outputs (in contrast to current behaviour); the user has to manually do that. This can be desirable behaviour as it removes the naming hack (haskellProjects.foo -> packages.foo-whatever) and forces the user to come up with a good name for their package output.

[srid]

Or:

{  # In perSystem
  haskell-flake = {
    projects.default = { .. };
    autoWire = true;
  };
}

(Assuming it is possible to wire flake outputs from inside of a perSystem config .... if it even makes sense 🤔)