When you pick up or craft an item required to craft a backpack (that isn't an upgrade of another backpack), the recipe is given to the player and displayed in the recipe book. When a backpack requires another backpack in the crafting grid, the recipe is unlocked once you craft that specific backpack. It wasn't as hard as I thought to implement.

+ fixed a small bug that it didn't work using a backpack in multiple recipes (now you can have multiple upgrade-backpacks for one backpack)