merge: remove direct rustmod reference We shouldn't rely on this member being present in `dirstate.py`, this creates unnecessary coupling. This also can trigger certain issues in edge-cases where the policy is changed at runtime or multiple Python environments fight, which is an added bonus. Differential Revision: https://phab.mercurial-scm.org/D12217

graft c-ext
graft debian
graft zstd
graft tests
include make_cffi.py
include setup_zstd.py
include zstd.c
include LICENSE
include NEWS.rst