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).
@huoxito i've addressed the product assembly piece down below, let me know what you think.

As far as the inventory units being built here and the state being set later, that allows us to build the necessary inventory units separately from deciding what shipment they go into (and their shipment/stock location is what determines they are on hand or backordered).

If you think another service object would be better for inventory unit building so that it can be more easily configured to enable the building of custom packages where the line item and variant don't match, I think that would be a fine design choice.