Fix N+1 on Shipment#to_package call to variant from Differentiator

On checkout/delivery page, a page refresh calls out to the
`Stock::Differentiator`, which loads a variant from each `inventory_unit`

The fix eliminates N+1 sql select for each variant.

Ex:

```sql
  CACHE (0.0ms)  SELECT  "spree_variants".* FROM "spree_variants"  WHERE "spree_variants"."id" = $1 LIMIT 1  [["id", 1]]
  CACHE (0.0ms)  SELECT  "spree_variants".* FROM "spree_variants"  WHERE "spree_variants"."id" = $1 LIMIT 1  [["id", 1]]
  CACHE (0.0ms)  SELECT  "spree_variants".* FROM "spree_variants"  WHERE "spree_variants"."id" = $1 LIMIT 1  [["id", 1]]
```