extra_path is implicated in [this failure](https://github.com/jaraco/rwt/issues/7) and in general in any attempt to install to a path in PYTHONPATH that's not in site-packages. For example:

$ python -m pip install -t foo newrelic
Collecting newrelic
Installing collected packages: newrelic
Successfully installed newrelic
$ ls foo
newrelic-2.68.0.50		newrelic-2.68.0.50.dist-info	newrelic.pth
$ PYTHONPATH=foo python -c "import newrelic"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named 'newrelic'


As a result, extra_path (undocumented per issue901727) causes disruption in tools like rwt, and gives the packager the ability to break the convention for installing the package, relying on .pth files which only work in select locations.

I suggest we deprecate extra_path.