fix(medusa) : Improperly referenced product id in product import strategy (#2351)

* What - fixing PR #2350 

* Why - In the ProductImportStrategy class, the getImportInstructions improperly checks the product.id for each row. With it setup this way, products will always be considered new because the product.id column is not read here. So products will never be updated. [Link to the code line](https://github.com/medusajs/medusa/blob/bd941309161bcee4fbfc7918d310c426ba8d8f15/packages/medusa/src/strategies/batch-jobs/product/import.ts#L166)

* How - Replacing `row["product.product.id"]` to  `row["product.id"]`

Co-authored-by: Oliver Windall Juhl <59018053+olivermrbl@users.noreply.github.com>

.changeset/khaki-pets-repeat.md

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+---
+"@medusajs/medusa": patch
+---
+
+Fix wrongly referenced product id in ProductImportStrategy
+Fix assigning nested objects properly