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

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Greg Ward, author of the original bfiles extension

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Na'Tosha Bard of Unity Technologies

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Fog Creek Software

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Special thanks to the University of Toronto and the UCOSP program