>I don't know how common ghostwriting is for songs; it's common for artists to record songs written by someone else, but I'm not sure how common it is for artists to use a ghostwriter for that without giving any credit at all.

Extraordinarily common. I work in the music business and see it first hand. Maybe half of charting pop songs written in the last 20 years, are ghostwritten. "Hot and Cold" by Katy Perry (credited as Perry, Max Martin and Dr Luke) was actually written about my friend by his ex (not credited). That's not n=1 it's just an example to give you a flavor