ENH: properly handle egg-info handling for all cases.

We now distinguish between 3 cases:

    - for a non Enthought-built egg, we copy the content of EGG-INFO
      into $site-packages/$package.egg-info
    - for an Enthought-built egg built with egg_info_as_dir=False passed
      to workbench.EggCreator (the default), we copy the EGG-INFO
      content into the correct location inside site-packages, with a
      blacklist to ignore Enthought-specific bits.
    - for an Enthought-built egg built with egg_info_as_dir=True passed
      to workbench.EggCreator, we move the egg-info content into the
      correct location, and don't copy anything from EGG-INFO.