Disambiguate distributions installed in generic site-packages path during pex_bootstrap.

If you pip install twitter.common.python using sufficiently new pips, it
will get installed into site-packages as an EGG-INFO distribution instead of
an .egg.  This means its dist.location is site-packages instead of
site-packages/setuptools.egg, so interpreter.get_location('setuptools') is
ambiguous.  This removes the ambiguity by adding a name= keyword to
DistributionHelper.distribution_from_path.

Reviewed at https://rbcommons.com/s/twitter/r/249/