package "interface_adapters.browse" renamed as "interface_adapters.add_to_cart"

[nehmat-kaur]

The interface adapters are supposed to be corresponding to the views, not the use cases. "add_to_cart" is a use case, while "browse" is the view where this use case is being implemented. But now that it has been renamed, let's just keep it this way for convenience, and we can refractor it at the end. This issue is just to avoid any confusion and point out that add_to_cart is not a view, it is a use_case.