Ubercart must not use hook_add_to_cart() - Change to hook_ubercart_add_to_cart() (and namespace any other hooks)

The accepted behaviors for contrib modules is that they have to namespace their hooks. Thus, ubercart should be using hook_ubercart_add_to_cart as a hook, not hook_add_to_cart.

There are other carts in Drupal... There are other "add to cart" modules. There are other functions called add_to_cart(). It's important to change all hooks to include the ubercart namespace.

This problem was responsible for #494304: Add to Ubercart Cart creates Amazon Cart error messages in Amazon Store module. Amazon store had an innocent amazon_store_add_to_cart() function, which unfortunately got called when Ubercart did an add to cart.

Thanks,
-Randy