vfs: use 'vfs' module directly in 'mercurial.hg'
Now that the 'vfs' classes moved in their own module, lets use the new module
directly. We update code iteratively to help with possible bisect needs in the
future.
# name space package to host third party extensionsfrom __future__ import absolute_importimport pkgutil__path__ = pkgutil.extend_path(__path__, __name__)