Non zipsafe dependencies for plugins aren't extracted out of eggs

[toddgardner]

If you create a plugin that requires a non-zipsafe dependency, pants will attempt to run it from an egg, which messes up packages like boto and boto3:

  File "build/bdist.macosx-10.12-x86_64/egg/boto3/session.py", line 347, in resource
    has_low_level_client)

Exception message: The 's3' resource does not exist.
The available resources are:
   - 

These need to be extracted to the local filesystem before being used.

As a workaround, these dependencies can be installed into the pants virtualenv directly by modifying the pants setup script.

[jsirois]

Sorry for the large delay getting to this @toddgardner. It looks like this should have been solved by #4887 which flipped plugin installation to using wheels which are properly installed on the filesystem instead of being accessed through their zipped .whl file. I'll mark this as closed but please speak up if you find otherwise.