title: Downgrading pip fails on virtualenv
body: <p>I'm working on Ubuntu 16.04 with python2.7 virtualenv (version 15.0.1).
  When I create a new virtualenv it automatically installs pip (version 8.1.1).
  I want to downgrade pip to version 8.0.2 but when running </p>
  
  <pre><code>pip install pip==8.0.2
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>Exception:
  Traceback (most recent call last):
    File "/mnt/code/.venvs/bla/local/lib/python2.7/site-packages/pip/basecommand.py", line 209, in main
      status = self.run(options, args)
    File "/mnt/code/.venvs/bla/local/lib/python2.7/site-packages/pip/commands/install.py", line 328, in run
      wb.build(autobuilding=True)
    File "/mnt/code/.venvs/bla/local/lib/python2.7/site-packages/pip/wheel.py", line 748, in build
      self.requirement_set.prepare_files(self.finder)
    File "/mnt/code/.venvs/bla/local/lib/python2.7/site-packages/pip/req/req_set.py", line 360, in prepare_files
      ignore_dependencies=self.ignore_dependencies))
    File "/mnt/code/.venvs/bla/local/lib/python2.7/site-packages/pip/req/req_set.py", line 448, in _prepare_file
      req_to_install, finder)
    File "/mnt/code/.venvs/bla/local/lib/python2.7/site-packages/pip/req/req_set.py", line 387, in _check_skip_installed
      req_to_install.check_if_exists()
    File "/mnt/code/.venvs/bla/local/lib/python2.7/site-packages/pip/req/req_install.py", line 1011, in check_if_exists
      self.req.project_name
  AttributeError: 'Requirement' object has no attribute 'project_name'
  </code></pre>
  
  <p>However, running it outside the virtualenv works.</p>
  
  <p>Does anyone know a solution to this problem?</p>
  
  <p>The reason I want to downgrade the pip version is because the newer version converts dots <code>(.)</code> to dashes <code>(-)</code> when searching for packages, and it doesn't find some of the packages I need.</p>