manifest: introduce memmanifestctx and memtreemanifestctx
This introduces two new classes to represent in-memory manifest instances.
Similar to memchangectx, this lets us prepare a manifest in memory, then in a
future patch we will add the apis that can commit this in memory structure.
# name space package to host third party extensions
from __future__ import absolute_import
import pkgutil
__path__ = pkgutil.extend_path(__path__, __name__)