spotify: avoid librespot::discovery dependency

librespot::discovery::Credentials is a re-export of librespot::core::authentication::Credentials. We must depend on librespot::core so we can get it from there and avoid also depending on librespot::discovery just for it. There's currently no material benefit in this... but it's the right thing to do? And might help us later if librespot makes extra modules like this optional...