shelve: move function for opening .patch file to new shelf class
The `opener()` method was used specifically for the `.patch` file, and
the new `Shelf` class deals with all files involved in a shelf, so I
renamed the function in the process.
Differential Revision: https://phab.mercurial-scm.org/D9705
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)