ghcjs: refactor stage2 handling

Previously, the stage2 information was stored in haskell-modules, and imported directly from there. However, the correct stage2 information is determined by the version of ghcjs-boot repository. This commit makes the stage2 information part of the ghcjs derivation, which improves organization and makes it possible to override stage2 when overriding ghcjs.

pkgs/development/compilers/ghcjs/default.nix

···
     isGhcjs = true;
     inherit nodejs ghcjsBoot;
     inherit (ghcjsNodePkgs) "socket.io";
+    mkStage2 = import ./stage2.nix {
+      inherit ghcjsBoot;
+    };
   };
 
   homepage = "https://github.com/ghcjs/ghcjs";

pkgs/development/compilers/ghcjs/gen-stage2.rb

···
+#!/usr/bin/env ruby
+
+require 'pathname'
+
+# from boot.yaml in ghcjs/ghcjs
+stage2_packages = [
+  "boot/async",
+  "boot/aeson",
+  "boot/attoparsec",
+  "boot/case-insensitive",
+  "boot/dlist",
+  "boot/extensible-exceptions",
+  "boot/hashable",
+  "boot/mtl",
+  "boot/old-time",
+  "boot/parallel",
+  "boot/scientific",
+  "boot/stm",
+  "boot/syb",
+  "boot/text",
+  "boot/unordered-containers",
+  "boot/vector",
+  "ghcjs/ghcjs-base",
+  # not listed under stage2, but needed when "quick booting".
+  "boot/cabal/Cabal"
+]
+
+nixpkgs = File.expand_path("../../../../..", __FILE__)
+boot = ARGV[0] || `nix-build #{nixpkgs} -A haskell.packages.ghcjs.ghc.ghcjsBoot`.chomp
+
+out = "".dup
+out << "{ ghcjsBoot }: { callPackage }:\n"
+out << "\n"
+out << "{\n"
+
+stage2_packages.each do |package|
+  name = Pathname.new(package).basename
+  nix = `cabal2nix file://#{boot}/#{package}  --jailbreak`
+  nix.sub!(/src =.*?$/, "src = \"${ghcjsBoot}/#{package}\";")
+  nix.sub!("  doCheck = false;\n", "")
+  nix.sub!("libraryHaskellDepends", "doCheck = false;\n  libraryHaskellDepends")
+  # cabal2nix somehow generates the deps for 'text' as if it had selected flag
+  # 'integer-simple' (despite not passing the flag within the generated
+  # expression). We want integer-gmp instead.
+  nix.gsub!(/integer-simple/, "integer-gmp")
+  nix = nix.split("\n").join("\n      ")
+
+  out << "  #{name} = callPackage\n"
+  out << "    (#{nix}) {};\n"
+end
+
+out << "}"
+
+puts out

pkgs/development/haskell-modules/configuration-ghcjs.nix

···
 
 self: super:
   # The stage 2 packages. Regenerate with ./ghcjs/gen-stage2.rb
-  let stage2 =
-    (import ./ghcjs/stage2.nix {
+  let stage2 = super.ghc.mkStage2 {
        inherit (self) callPackage;
-       inherit (self.ghc) ghcjsBoot;
-    }); in stage2 // {
+    }; in stage2 // {
 
   old-time = overrideCabal stage2.old-time (drv: {
     postPatch = ''

pkgs/development/haskell-modules/ghcjs/gen-stage2.rb

···
-#!/usr/bin/env ruby
-
-require 'pathname'
-
-# from boot.yaml in ghcjs/ghcjs
-stage2_packages = [
-  "boot/async",
-  "boot/aeson",
-  "boot/attoparsec",
-  "boot/case-insensitive",
-  "boot/dlist",
-  "boot/extensible-exceptions",
-  "boot/hashable",
-  "boot/mtl",
-  "boot/old-time",
-  "boot/parallel",
-  "boot/scientific",
-  "boot/stm",
-  "boot/syb",
-  "boot/text",
-  "boot/unordered-containers",
-  "boot/vector",
-  "ghcjs/ghcjs-base",
-  # not listed under stage2, but needed when "quick booting".
-  "boot/cabal/Cabal"
-]
-
-nixpkgs = File.expand_path("../../../../..", __FILE__)
-boot = `nix-build #{nixpkgs} -A haskell.packages.ghcjs.ghc.ghcjsBoot`.chomp
-
-out = "".dup
-out << "{ ghcjsBoot, callPackage }:\n"
-out << "\n"
-out << "{\n"
-
-stage2_packages.each do |package|
-  name = Pathname.new(package).basename
-  nix = `cabal2nix file://#{boot}/#{package}  --jailbreak`
-  nix.sub!(/src =.*?$/, "src = \"${ghcjsBoot}/#{package}\";")
-  nix.sub!("libraryHaskellDepends", "doCheck = false;\n  libraryHaskellDepends")
-  # cabal2nix somehow generates the deps for 'text' as if it had selected flag
-  # 'integer-simple' (despite not passing the flag within the generated
-  # expression). We want integer-gmp instead.
-  nix.gsub!(/integer-simple/, "integer-gmp")
-  nix = nix.split("\n").join("\n      ")
-
-  out << "  #{name} = callPackage\n"
-  out << "    (#{nix}) {};\n"
-end
-
-out << "}"
-
-puts out

pkgs/development/haskell-modules/ghcjs/stage2.nix pkgs/development/compilers/ghcjs/stage2.nix

···
-{ ghcjsBoot, callPackage }:
+{ ghcjsBoot }: { callPackage }:
 
 {
   async = callPackage