haskellPackages.shellFor missing deps from .extend

[luigy]

Issue description

Steps to reproduce

$ cat default.nix 
with (import <nixpkgs> {});
with haskell.lib;
rec {
  hsPkgs =
    (haskellPackages.extend (packageSourceOverrides { foo = ./.; }))
      .extend (self: super: { foo = addBuildDepend super.foo self.groom; });
  hsShell = hsPkgs.shellFor {
    packages = p: [ p.foo ];
    withHoogle = false;
  };
}

$ cat foo.cabal
name: foo
version: 0.5.0
library
  build-depends:
      base

$ nix-shell -A hsPkgs.foo.env --run "ghc-pkg list | grep groom"
    groom-0.1.2.1

$ nix-shell -A hsShell --run "ghc-pkg list | grep groom"

Technical details

• system: "x86_64-linux"
• host os: Linux 4.15.16, NixOS, 18.03.git.2569e48 (Impala)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.0
• channels(luigy): "nixpkgs-18.09pre139198.9d0b6b9dfc9"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs

[luigy]

cc @ElvishJerricco