do not walk on whole package for egg-info dir

If only the top level *.egg-info dir is going to be picked,
then there is no need to walk after the first one is found
because os.walk() runs in top-down direction.

I did this because, we are deploying our application via git then installing in editable mode with "pip install -e .". Since there are thousands of small files and directories in our package, install command takes too much time, especially when there is a heavy load on the server.

I run the tests and these 2 have passed after my patch:

```
cenk@aslan ~/projects/pip/tests (develop*) $ nosetests -s -v test_basic.py:test_install_from_local_directory test_basic.py:test_download_editable_to_custom_path
Test installing from a local directory. ... ok
Test downloading an editable using a relative custom src folder. ... ok

----------------------------------------------------------------------
Ran 2 tests in 4.799s

OK
```

I cannot run all of them because Travis-CI is failing currently. I think these 2 should cover my patch.