title: Pip list raises ValueError
body: <p>I am not able to run <code>$ pip list</code> using pip 19.0.1 from /usr/local/lib/python3.6/dist-packages/pip (python 3.6). </p>
  
  <pre><code>$ pip list
  Exception:
  Traceback (most recent call last):
    File "/usr/local/lib/python3.6/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 2584, in version
      return self._version
    File "/usr/local/lib/python3.6/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 2691, in __getattr__
      raise AttributeError(attr)
  AttributeError: _version
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/usr/local/lib/python3.6/dist-packages/pip/_internal/cli/base_command.py", line 176, in main
      status = self.run(options, args)
    File "/usr/local/lib/python3.6/dist-packages/pip/_internal/commands/list.py", line 148, in run
      self.output_package_listing(packages, options)
    File "/usr/local/lib/python3.6/dist-packages/pip/_internal/commands/list.py", line 205, in output_package_listing
      data, header = format_for_columns(packages, options)
    File "/usr/local/lib/python3.6/dist-packages/pip/_internal/commands/list.py", line 271, in format_for_columns
      row = [proj.project_name, proj.version]
    File "/usr/local/lib/python3.6/dist-packages/pip/_vendor/pkg_resources/__init__.py", line 2589, in version
      raise ValueError(tmpl % self.PKG_INFO, self)
  ValueError: ("Missing 'Version:' header and/or METADATA file", Unknown [unknown version] (/usr/local/lib/python3.6/dist-packages))
  </code></pre>
  
  <p>I'm working on a fresh virtual machines where these commands have been run, with the final command installing something I had cloned from github.</p>
  
  <pre><code>apt-get install -y python3 python3-pip python3-dev
  python3 -m pip install --upgrade pip
  pip3 install https://github.com/explosion/spacy-models/releases/download/en_core_web_sm-2.0.0/en_core_web_sm-2.0.0.tar.gz#egg=en_core_web_sm-2.0.0
  sudo -H pip3 install -e /vagrant
  </code></pre>