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
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