Issue description

pipenv commands that use pip internally or explicitly fail.

Steps to reproduce

1. make sure we're in a pristine and safely disposable directory

   cd $(mktemp -d)

2. run a pipenv command that uses pip explicitly

   nix-shell -p pipenv --run 'pipenv run pip --version'

Expected

• Virtualenv is created
• Pipfile is created
• Version of pip in that virtualenv is displayed

Observed

• ✔️ Virtualenv is created

   Creating a virtualenv for this project…
   ⠋Using base prefix '/nix/store/ljhgdba6n8ag6f8clpi4m9zizm7b8mx3-python3-3.6.5'
   New python executable in /home/das-g/.local/share/virtualenvs/tmp.btSs5gvz0u-HGofKOag/bin/python3.6m
   Also creating executable in /home/das-g/.local/share/virtualenvs/tmp.btSs5gvz0u-HGofKOag/bin/python
   Installing setuptools, pip, wheel...done.
   Virtualenv location: /home/das-g/.local/share/virtualenvs/tmp.btSs5gvz0u-HGofKOag
  

• ✔️ Pipfile is created

  Creating a Pipfile for this project…
  

• ✖️ Then, a ModuleNotFoundError error is displayed:

  Traceback (most recent call last):
    File "/home/das-g/.local/share/virtualenvs/tmp.btSs5gvz0u-HGofKOag/bin/pip", line 7, in <module>
      from pip._internal import main
  ModuleNotFoundError: No module named 'pip._internal'
  

Cleanup after reproducing

In this order:

1. Remove the virtualenv:

   nix-shell -p pipenv --run 'pipenv --rm'

2. cd - and remove the temporary directory with its content

Variations

Similar observations for

• pipenv commands using pip internally, e.g. pipenv install requests
• invoking pip in a pipenv shell:

  nix-shell -p pipenv --run 'pipenv shell'
  pip --version

• doing any of the above in an interactive nix-shell with pipenv

Technical details

• system: "x86_64-linux"
• host os: Linux 4.14.35, NixOS, 18.03.132083.06c576b0525 (Impala)
• multi-user?: yes
• sandbox: no
• version: nix-env (Nix) 2.0
• channels(root): "nixos-18.03.132083.06c576b0525"
• channels(das-g): ""
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs