only stage OS X readline if it is an extension

Custom readline can be readline.py, which shouldn't be staged as readline.so.

Since pip doesn't do the sys.path shenanigans that easy_install does, the readline project is considering a change where the extension is renamed to `gnureadline.so`, since there is no way to override a stdlib module with pip. The proposed change adds a `readline.py` that does `from gnureadline import *` for people who still expect the easy_install behavior.

With this change, `imp.find_module('readline')` would find `readline.py`, which is currently being staged as `readline.so` on OS X, which will not be importable, failing with:

```python
ImportError: dlopen(VIRTUAL_ENV/lib/python2.7/readline.so, 2): no suitable image found.  Did find:
	VIRTUAL_ENV/lib/python2.7/readline.so: file too short
```

This PR simply excludes non-extension readline from the special handling.

Only easy_installed readline is affected, since pip installed readline will never be found by `find_module('readline')`.

The long-term results of this is that users who install a site-packages readline will never have that package staged into virtualenv.