stack: update default version to 1.6.1

Allow building with either ghc 8.0.x or 8.2.x.

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

···
   hpack_0_20_0 = dontCheck super.hpack_0_20_0;
   hpack_0_21_0 = dontCheck super.hpack_0_21_0;
 
+  # Stack 1.6.1 needs newer versions than LTS-9 provides.
+  stack = super.stack.overrideScope (self: super: {
+    ansi-terminal = self.ansi-terminal_0_7_1_1;
+    ansi-wl-pprint = self.ansi-wl-pprint_0_6_8_1;
+    extra = dontCheck super.extra_1_6_2;
+    hpack = super.hpack_0_20_0;
+    path = dontCheck super.path_0_6_1;
+    path-io = self.path-io_1_3_3;
+    unliftio = self.unliftio_0_2_0_0;
+  });
+
 }

pkgs/development/haskell-modules/configuration-ghc-8.0.x.nix

···
   # This builds needs the latest Cabal version.
   cabal2nix = super.cabal2nix.overrideScope (self: super: { Cabal = self.Cabal_2_0_1_1; });
 
+  # Add appropriate Cabal library to build this code.
+  stack = addSetupDepend super.stack self.Cabal_2_0_1_1;
+
 }