handle_liquid needs to be refactored

[mlangsdorf]

As written, handle_liquid does two different things: selects the container for a liquid transfer, and performs the transfer. It would be helpful to split those functions into two different functions: one that
selects the container, the other that performs the transfer. Other code that wanted to transfer liquid
in different ways would be able to call either function, as needed.
handle_liquid would remain as a wrapper that calls the two functions in sequence, so existing code
wouldn't need to change.

I'm going to go write this so I can create a MILKING activity.