revlog: move the details of revlog "v2" index inside revlog.utils.constants
the revlog module is quite large and this kind of format information would handy
for other module. So let us start to gather this information about the format in
a more appropriate place.
We update various reference to this information to use the new "source of truth"
in the process.
Differential Revision: https://phab.mercurial-scm.org/D10305
public attribute not declared in interfaces: badpeer.badattribute
public attribute not declared in interfaces: badpeer.badmethod