Add option to manually specify package set overlays instead of providing settings option

[eldritch-cookie]

The Problem

I commonly want to develop my projects with the latest ghc version, this means that i need to apply fixes for many packages while i wait for the fixes i upstreamed to land. However this is extremely painful to do currently with haskell-flake.

There are various issues which contribute to this. The settings option is really underdocumented so whenever i need to apply a fix i am forced to go to the haskell flake source code at nix/modules/settings/all.nix, this also means that i also NEED to already know how to apply the desired fix via the functions on pkgs.haskell.lib. Even worse if i need a version change or anything uncommon i am forced to use the custom attribute, which means that i need to rewrite all my extra fixes to be in the overlay version to guarantee they aren't discarded.

additionally having 2 different methods of applying fixes means that i when want to upstream mine i have to convert everything to be nixpkgs compatible which may be like a minute per package but that means almost 2 hours if have 100 packages, which isn't uncommon when i first upgrade to a new ghc version.

Proposed solution

Instead of having a settings option which reimplements everything on pkgs/development/haskell-modules/lib/compose.nix just give an option where the user provides a haskell package set overlay, the ergonomics then are equivalent to nixpkgs and no duplication is necessary.

[srid]

You can use otherOverlays for this:

haskell-flake/nix/modules/project/default.nix

Lines 71 to 81 in b3511b3

	otherOverlays = lib.mkOption {
	type = types.listOf (import ../../types/haskell-overlay-type.nix { inherit lib; });
	description = ''
	Extra overlays to apply.
	Normally, you would only use `packages.*` and `settings.*` (which
	translate to overlays), but you can use this option if you want control
	over the final overlay.
	'';
	default = [ ];
	};

PR welcome on documentation!