Comment 7 for bug 1586381

This bug also seems to makes it impossible to update pip to a specific version using `pip install --upgrade pip==<version>` or `pip install <pip-package-url>` or `pip install <pip-package-file>`.

E.g:

```
# pip install pip-8.1.2-py2.py3-none-any.whl
Exception:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/pip/basecommand.py", line 209, in main
    status = self.run(options, args)
  File "/usr/lib/python2.7/dist-packages/pip/commands/install.py", line 328, in run
    wb.build(autobuilding=True)
  File "/usr/lib/python2.7/dist-packages/pip/wheel.py", line 748, in build
    self.requirement_set.prepare_files(self.finder)
  File "/usr/lib/python2.7/dist-packages/pip/req/req_set.py", line 360, in prepare_files
    ignore_dependencies=self.ignore_dependencies))

  File "/usr/lib/python2.7/dist-packages/pip/req/req_set.py", line 448, in _prepare_file
    req_to_install, finder)
  File "/usr/lib/python2.7/dist-packages/pip/req/req_set.py", line 387, in _check_skip_installed
    req_to_install.check_if_exists()
  File "/usr/lib/python2.7/dist-packages/pip/req/req_install.py", line 1011, in check_if_exists
    self.req.project_name
AttributeError: 'Requirement' object has no attribute 'project_name'
You are using pip version 8.1.1, however version 8.1.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
```

Only `pip install --upgrade pip` (with no version constraint) works as expected.

For deploys on machines which don't have internet access (and thus can't access PyPI directly) this problem is very inconvenient.