mergeInputs is now simply defined in terms of `concatLists` and
`catAttrs` instead of a more complicated `foldr`.

Note that the order of PATH has also changed. For example running the
following with nix-shell:

  let
    pkgs = import <nixpkgs> {};

    shell1 = pkgs.mkShell {
      buildInputs = [ pkgs.htop ];
    };

    shell2 = pkgs.mkShell {
      buildInputs = [ pkgs.hello ];
    };

    shell3 = pkgs.mkShell {
      inputsFrom = [ shell1 shell2 ];
      buildInputs = [ pkgs.tree ];
    };

  in shell3

Results in the following PATH:

$ echo $PATH
...
/nix/store/yifq4bikf7m07160bpia7z48ciqddbfi-tree-1.8.0/bin:
/nix/store/vhxqk81234ivqw1a7j200a1c69k8mywi-htop-2.2.0/bin:
/nix/store/n9vm3m58y1n3rg3mlll17wanc9hln58k-hello-2.10/bin
...

Previously the order was:

/nix/store/n9vm3m58y1n3rg3mlll17wanc9hln58k-hello-2.10/bin
/nix/store/vhxqk81234ivqw1a7j200a1c69k8mywi-htop-2.2.0/bin:
/nix/store/yifq4bikf7m07160bpia7z48ciqddbfi-tree-1.8.0/bin:

I think the new order makes more sense because it allows to override
the PATH in the outermost mkShell.

(cherry picked from commit cee3573)

Running the following expression with nix-shell:

  let
    pkgs = import <nixpkgs> {};

    shell1 = pkgs.mkShell {
      shellHook = ''
        echo shell1
      '';
    };

    shell2 = pkgs.mkShell {
      shellHook = ''
        echo shell2
      '';
    };

    shell3 = pkgs.mkShell {
      inputsFrom = [ shell1 shell2 ];
      shellHook = ''
        echo shell3
      '';
    };
  in shell3

Will now results in:
shell2
shell1
shell3

Note that packages in the front of inputsFrom have precedence over
packages in the back. The outermost mkShell has precedence over all.

(cherry picked from commit 76ef802)