revlog: move the nodemap into the index object (for pure) This make the pure code closer to the C extension one. The ultimate goal is to merge the two into a single object and offer a unified API. This changeset focus on gathering the data on the same object. For now the code for `revlogoldindex` and `BaseIndexObject` index object are quite similar. However, there will be larger divergence later on, so I don't think is worth doing a base case. This work is part of a refactoring to unify the revlog index and the nodemap. This unification prepare the use of a persistent nodemap. Differential Revision: https://phab.mercurial-scm.org/D7313