refactor(3p/overlays): Move haskell overlay to new tree location

nixpkgs_exposed is going away, and the haskell overlay is independent
from that.

See also b/108, cl/2910

Change-Id: I3aea6dfc427a914f3f88146fd0b45d60dfd45a1a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2918
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/nixpkgs-exposed/exposed/default.nix

@@ -190,7 +190,9 @@
     ;
 
   haskellPackages = (nixpkgs.haskellPackages.override {
-    overrides = (import ../haskell_overlay { pkgs = nixpkgs; });
+    overrides = (import ../../overlays/haskell
+    { /* empty readTree arg */ }
+    { pkgs = nixpkgs; });
   });
 
   gradle_6 = (nixpkgs.gradleGen.override {

third_party/overlays/haskell/default.nix

@@ -1,4 +1,8 @@
-{ pkgs }:
+# Defines overrides for Haskell packages, for example to avoid
+# breakage currently present in nixpkgs or to modify package versions.
+
+{ ... }: # This file needs nothing from readTree
+{ pkgs }: # ... but is called with a separate package set in the overlay
 
 self: super: with pkgs.haskell.lib; rec {
   generic-arbitrary = appendPatch