Do the same transformation to egg_info dirs that pkg_resources does

To determine whether we should re-install an egg, we look for an .egg-info
directory based on the output of `setup.py --name`, but .egg-info directories
have their dashes transformed into underscores by pkg_resources.to_filename.

When we look for an .egg-info directory, we should apply the same
transformation.

src/tox/venv.py

@@ -8,6 +8,7 @@
 from itertools import chain
 
 import py
+from pkg_resources import to_filename
 
 import tox
 
@@ -295,7 +296,7 @@ def _needs_reinstall(self, setupdir, action):
             sys_path = json.loads(out)
         except ValueError:
             sys_path = []
-        egg_info_fname = ".".join((name, "egg-info"))
+        egg_info_fname = ".".join((to_filename(name), "egg-info"))
         for d in reversed(sys_path):
             egg_info = py.path.local(d).join(egg_info_fname)
             if egg_info.check():