Make pip install -e uninstall existing versions

[xavfernandez]

Based on #1552

[xavfernandez]

cc @msabramo @qwcode

[minrk]

I'm really excited about this. The more pip install -e behaves like pip install, the better. This plus setuptools 25's change to stop editing sys.path is going to improve my life a whole bunch.

I can verify that this is working for me, especially since prior editable installs have caused a problem where uninstall fails, due to an inverted priority:

(test-pip) benjaminrk[17:58]~/dev/py/pip (65a82a2) $ pip install .
Processing /Users/benjaminrk/dev/py/pip
Installing collected packages: pip
  Found existing installation: pip 8.1.2
Exception:
Traceback (most recent call last):
  File "/Users/benjaminrk/env/test-pip/lib/python3.5/site-packages/pip/basecommand.py", line 215, in main
    status = self.run(options, args)
  File "/Users/benjaminrk/env/test-pip/lib/python3.5/site-packages/pip/commands/install.py", line 317, in run
    prefix=options.prefix_path,
  File "/Users/benjaminrk/env/test-pip/lib/python3.5/site-packages/pip/req/req_set.py", line 736, in install
    requirement.uninstall(auto_confirm=True)
  File "/Users/benjaminrk/env/test-pip/lib/python3.5/site-packages/pip/req/req_install.py", line 687, in uninstall
    '(at %s)' % (link_pointer, self.name, dist.location)
AssertionError: Egg-link /Users/benjaminrk/dev/py/pip does not match installed location of pip (at /Users/benjaminrk/env/test-pip/lib/python3.5/site-packages)

[sigmavirus24]

This looks fine to me. I'm not sure the behaviour is exactly what I'd expect but shrug.

[fungi]

The OpenStack community would love to see this (or something) merged to solve the editable installs problem introduced by setuptools 25.0.0. We only just now discovered that projects using tox with usedevelop=True in our CI have not actually been testing the source code we expected if something else has already dragged in a packaged version of them as a transitive dependency.