Clear the Product -> Line item cache on Product Variation Type update

IRC conversation explaining this situation and potential solution:

15:12 bojanz: joshmiller: Look at \Drupal\commerce_product\LineItemTypeMap
15:12 bojanz: joshmiller: it caches this relationship: Product type -> Product variation type -> Line item type
15:13 bojanz: joshmiller: so if you ask for the line item type for a product type, it gives you one, without needing to load the first two config entities
15:13 joshmiller: ah, cool
15:13 bojanz: joshmiller: look at commerce_product_entity_bundle_create(), it clears that cache when a new product type is created
15:13 bojanz: (you can't edit the product type -> product variation type relationship afterwards)
15:14 bojanz: joshmiller: however, you can change the line item type specified on a product variation type
15:14 bojanz: joshmiller: so we need to write a hook that reacts on commerce_product_variation_type save
15:14 bojanz: joshmiller: if the save was an update, and the line item type changed, clear the cache
15:16 bojanz: joshmiller: there are no D8 concepts in this task :)
15:17 joshmiller: yay! hooks!
15:20 bojanz: joshmiller: now, I am adding a complication
15:20 bojanz: joshmiller: instead of using a normal hook, we'll use the method version of it
15:20 bojanz: joshmiller: if you create a public function postSave(EntityStorageInterface $storage, $update = TRUE) { on the ProductVariationType, it will function the same as a post save hook
15:21 bojanz: joshmiller: $update tells you whether the entity was modified or created, and $this->original->lineItemType gives you the original value to compare against
15:21 bojanz: joshmiller: Commerce already has examples of such postSave() methods, so you can look at them
15:22 bojanz: (the actual line for clearing the cache remains the same as in commerce_product_entity_bundle_create)