pipenv: pipenv thinks propagatedBuildInputs packages are installed in all environments

[Kaali]

Issue description

When running pipenv, it resolved site-packages as installed packages by default. Which means that when installing packages with pipenv it thinks every package included in PYTHONPATH is already installed, and does not install them.

When launching pipenv shell/virtualenv nix provided PYTHONPATH for pipenv wrapper is gone, which means that the packages it thought were installed, is not really there.

Steps to reproduce

1. Install pipenv with nix
2. Create a Pipfile with a dependency for idna
3. Install Pipfile and open a pipenv shell
4. Try to import idna
5. Notice that idna is not installed, even though pipenv graph thinks it is
6. Double check that non pipenv wrapped command pip check also knows that the packagei

Technical details

Please run nix-shell -p nix-info --run "nix-info -m" and paste the
results.

 - system: `"x86_64-darwin"`
 - host os: `Darwin 18.5.0, macOS 10.14.4`
 - multi-user?: `no`
 - sandbox: `no`
 - version: `nix-env (Nix) 2.2.1`
 - channels(kaali): `"nixpkgs-19.09pre172680.92ec809473e"`
 - nixpkgs: `/Users/kaali/dev/nix/nixpkgs`

[Kaali]

Issue resolved with merged PR.