Move pkg/billing into autoscaler-agent

[sharnoff]

Spun off from #1078 (comment)

@Omrigan

Does it make sense to get rid of [pkg/billing] and move all the stuff to either agent/billing or reporting?

@sharnoff

I'd like to do that, yeah. Wanted to leave it outside the scope of this change for now though — will keep this comment open as a reminder.

Basically what the comment says — much of the functionality has been moved out of pkg/billing. All that's left is just moving the type definitions and some final utilities.