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
public attribute not declared in interfaces: badpeer.badattribute
public attribute not declared in interfaces: badpeer.badmethod