Add packageSourceOverrides function to haskell package sets

pkgs/development/haskell-modules/make-package-set.nix

···
     # Creates a Haskell package from a source package by calling cabal2nix on the source.
     callCabal2nix = name: src: self.callPackage (self.haskellSrc2nix { inherit src name; });
 
+    # : Map Name (Either Path VersionNumber) -> HaskellPackageOverrideSet
+    # Given a set whose values are either paths or version strings, produces
+    # a package override set (i.e. (self: super: { etc. })) that sets
+    # the packages named in the input set to the corresponding versions
+    packageSourceOverrides =
+      overrides: self: super: pkgs.lib.mapAttrs (name: src:
+        let isPath = x: builtins.substring 0 1 (toString x) == "/";
+            generateExprs = if isPath src
+                               then self.callCabal2nix
+                               else self.callHackage;
+        in generateExprs name src {}) overrides;
+
     ghcWithPackages = selectFrom: withPackages (selectFrom self);
 
     ghcWithHoogle = selectFrom: