Rename _CHALKS to _ARTIFACT_INFO

[nettrino]

The _CHALKS key contains metadata about artifacts, some of which are part of the chalkmarks (those without _) and some of which are part of the per-artifact runtime info. They do no correspond to chalkmarks. We should probably rename them to prevent any confusion on behalf of the users

[viega]

Agreed this is an improvement, though I think we could leave _CHALKS as an alternate name in there for a while if people already are expecting it? How to communicate about deprecation...

[nettrino]

@viega agreed - we should also decide what our approach around deprecating keys is - for instance do we keep keys around for the next N major revisions? How do we declare them as deprecated in con4m? Tracking deprecation discussion in #204