Refactor stock related service objects to deal with inventory units.

Prior to this change, stock related service objects (coordinator, packer,
prioritizer, package, adjuster) were tightly coupled to orders and line
items, such that they would only create a set of shipments for the set
of line items for an order.

This change makes those objects work with inventory units, which enables
future enhancements that would create shipments for an order that are
not necessarily tied to the initial purchase (e.g. upcoming exchanges
work).
@athal7 just so you know I think I'll, or someone else, put the order attribute back here at some point later. It seems useful to me if someone is working work some order specific logic with the package on calculators for example, any particular motivation to drop it here? (same for all related service objects too)