Refactor wiring in default.nix and nix/default.nix

Move sources, haskellNix and haskellNixOverlays to the top level
default.nix which allows for passing the haskellNixOverlays from
flake.nix and thus simply importing 'default.nix' directly.

The reason why this is important is that haskell.nix needs to use
a different overlay for when using in flake evaluationor normal one.
This is due to the fact that when evaluating flakes
builtins.currentSystem is not available.

default.nix

@@ -9,9 +9,16 @@
 { system ? builtins.currentSystem
 , crossSystem ? null
 , config ? { allowUnfreePredicate = (import ./nix/lib/unfree.nix).unfreePredicate; }
-  # Overrides for niv
 , sourcesOverride ? { }
-, packages ? import ./nix { inherit system crossSystem config sourcesOverride checkMaterialization enableHaskellProfiling; }
+, sources ? import ./nix/sources.nix { inherit system; } // sourcesOverride
+, haskellNix ? import sources."haskell.nix" {
+    sourcesOverride = {
+      hackage = sources."hackage.nix";
+      stackage = sources."stackage.nix";
+    };
+  }
+, haskellNixOverlays ? haskellNix.overlays
+, packages ? import ./nix { inherit system sources crossSystem config sourcesOverride haskellNixOverlays checkMaterialization enableHaskellProfiling; }
   # An explicit git rev to use, passed when we are in Hydra
   # Whether to check that the pinned shas for haskell.nix are correct. We want this to be
   # false, generally, since it does more work, but we set it to true in the CI

flake.nix

@@ -10,28 +10,12 @@
   outputs = { self, nixpkgs, haskell-nix, flake-utils, ... }:
     (flake-utils.lib.eachSystem [ "x86_64-linux" ] (system:
       let
-        #
-        # Obtain all niv sources extending it with the 'nixpkgs' flake input
-        #
-        sources = import ./nix/sources.nix { inherit system; };
-
-        #
-        # all packages from nix/default.nix
-        #
-        plutusPackages = import ./nix {
-          inherit system sources;
-          haskellNixOverlays = [ haskell-nix.overlay ];
-        };
-
-        #
-        # all packages from ./default.nix
-        #
         topLevel = import ./. {
           inherit system;
-          packages = plutusPackages;
+          haskellNixOverlays = [ haskell-nix.overlay ];
         };
 
-        inherit (plutusPackages) pkgs plutus ownOverlays;
+        inherit (topLevel) pkgs plutus ownOverlays;
         inherit (plutus) haskell iohkNix;
         inherit (plutus.lib) buildPursPackage buildNodeModules filterNpm gitignore-nix;
       in

shell.nix

@@ -1,9 +1,5 @@
-{ crossSystem ? null
-, system ? builtins.currentSystem
-, config ? { allowUnfreePredicate = (import ./nix/lib/unfree.nix).unfreePredicate; }
-, sourcesOverride ? { }
-, packages ? import ./. { inherit crossSystem config sourcesOverride enableHaskellProfiling; }
-, enableHaskellProfiling ? false
+{ system ? builtins.currentSystem
+, packages ? import ./. { inherit system; }
 }:
 let
   inherit (packages) pkgs plutus plutus-playground marlowe-playground plutus-pab marlowe-dashboard deployment docs;