generate overlay of built-in packages automatically

[cdepillabout]

In #12, I introduced a way for stacklock2nix to generate a completely new package set. Part of this includes creating an overlay that has the GHC boot packages set to null:

stacklock2nix/nix/build-support/stacklock2nix/default.nix

Lines 661 to 710 in 79f2b1f

	# An Nixpkgs Haskell overlay that has GHC boot packages set to `null`. This
	# is used as an initial overlay when creating a brand new package set.
	newPkgSetCompilerConfig = self: super: {
	# TODO: Should llvmPackages be enabled here?
	# llvmPackages = pkgs.lib.dontRecurseIntoAttrs self.ghc.llvmPackages;
	# Disable GHC core libraries.
	array = null;
	base = null;
	binary = null;
	bytestring = null;
	Cabal = null;
	containers = null;
	deepseq = null;
	directory = null;
	exceptions = null;
	filepath = null;
	ghc-bignum = null;
	ghc-boot = null;
	ghc-boot-th = null;
	ghc-compact = null;
	ghc-heap = null;
	ghc-prim = null;
	ghci = null;
	haskeline = null;
	hpc = null;
	integer-gmp = null;
	libiserv = null;
	mtl = null;
	parsec = null;
	pretty = null;
	process = null;
	rts = null;
	stm = null;
	template-haskell = null;
	# GHC only builds terminfo if it is a native compiler
	# terminfo = if pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform then null else self.terminfo_0_4_1_5;
	terminfo = null;
	text = null;
	time = null;
	transformers = null;
	unix = null;
	# GHC only bundles the xhtml library if haddock is enabled, check if this is
	# still the case when updating: https://gitlab.haskell.org/ghc/ghc/-/blob/0198841877f6f04269d6050892b98b5c3807ce4c/ghc.mk#L463
	# xhtml = if self.ghc.hasHaddock or true then null else self.xhtml_3000_2_2_1;
	xhtml = null;
	};

I think this overlay could likely be defined automatically, without it having to be hard-coded. Here's a function from Nixpkgs doing something similar:

NixOS/nixpkgs#217124