Improve our JEI integration a little bit

- Turtle and pocket computers provide a "creator mod id" based on their
   upgrade(s).
   We track which mod was active when the upgrade was registered, and
   use that to determine the owner. Technically we could use the
   RegistryLocation ID, but this is not always correct (such as
   Plethora's vanilla modules).
 - We show all upgraded turtles/pocket computers in JEI now, rather than
   just CC ones.
 - We provide a custom IRecipeRegistryPlugin for upgrades, which
   provides custom usage/recipes for any upgrade or upgraded item. We
   also hide our generated turtle/pocket computer recipes in order to
   prevent duplicates.

src/main/java/dan200/computercraft/api/pocket/IPocketUpgrade.java

@@ -54,6 +54,9 @@ public interface IPocketUpgrade
      * pocket computer which holds this upgrade. This item stack is also used to determine the upgrade given by
      * {@code pocket.equip()}/{@code pocket.unequip()}.
      *
+     * Ideally this should be constant over a session. It is recommended that you cache
+     * the item too, in order to prevent constructing it every time the method is called.
+     *
      * @return The item stack used for crafting. This can be {@link ItemStack#EMPTY} if crafting is disabled.
      */
     @Nonnull