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