Updated the haskell.nix imports to overlay style (#357)

* Updated the haskell.nix imports to overlay style

Some of the examples in the documentation were still passing an empty
set to the haskell.nix imports but this is no longer valid because the
import now results in a set rather than a function.

* Fix overlay imports in docs

Import an overlayed nixpkgs rather than just the overlay.
Thanks to angerman for noticing this error.

Author: toonn

docs/user-guide/cabal-projects.md

@@ -81,7 +81,9 @@ instantiate a package set.
 # default.nix
 let
   # Import the Haskell.nix library,
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
+  src = builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz;
+  nixpkgs = import (src + "/nixpkgs") (import src);
+  haskell = nixpkgs.haskell-nix;
 
   # Instantiate a package set using the generated file.
   pkgSet = haskell.mkCabalProjectPkgSet {

docs/user-guide/development.md

@@ -108,7 +108,9 @@ selects packages from the larger package set.
 ```nix
 # shell.nix
 let
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
+  src = builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz;
+  nixpkgs = import (src + "/nixpkgs") (import src);
+  haskell = nixpkgs.haskell-nix;
 in
   haskell.haskellPackages.ghcWithPackages (ps: with ps;
     [ lens conduit conduit-extra ])
@@ -125,7 +127,9 @@ project.
 
 ```nix
 let
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
+  src = builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz;
+  nixpkgs = import (src + "/nixpkgs") (import src);
+  haskell = nixpkgs.haskell-nix;
 in
   haskell.snapshots."lts-13.18".alex.components.exes.alex
 ```

docs/user-guide/stack-projects.md

@@ -43,7 +43,9 @@ instantiate a package set.
 # default.nix
 let
   # Import the Haskell.nix library,
-  haskell = import (builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz) {};
+  src = builtins.fetchTarball https://github.com/input-output-hk/haskell.nix/archive/master.tar.gz;
+  nixpkgs = import (src + "/nixpkgs") (import src);
+  haskell = nixpkgs.haskell-nix;
 
   # Instantiate a package set using the generated file.
   pkgSet = haskell.mkStackPkgSet {