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

Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program