Extra dependencies are listed in the Pypi JSON with the keyword "extra",
as such (example taken from https://pypi.org/pypi/celery/json):

    "cryptography ; extra == 'auth'",

This means that someone can run `pip install celery[auth]` and will get
the cryptography package installed.

But this scheme also allows for variations, as the following (from the
same URL as above):

    "couchbase-cffi (<3.0.0) ; (platform_python_implementation == \"PyPy\") and extra == 'couchbase'",

The current implementation of detection of "extra" dependencies does not
account for the second format, so I'm fixing it in this commit.