Preload variants when building inventory units to avoid an N+1

[ikraamg]

Building inventory units for an order's proposed shipments reads each line item's variant, and the variant's product when resolving its shipping category, with a separate query per line item. This runs whenever shipments are rebuilt, such as the delivery step of checkout.

• preloads each line item's variant and product before building the units
• variant and product loads stay constant regardless of order size
• adds a spec asserting line item variants load in a single query

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.69%. Comparing base (8d781ac) to head (ee909aa).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6498   +/-   ##
=======================================
  Coverage   89.68%   89.69%           
=======================================
  Files         993      993           
  Lines       20863    20864    +1     
=======================================
+ Hits        18712    18713    +1     
  Misses       2151     2151           

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