Every other core interface is named for what it is -- Playlists,
Library, Scrobbler, MediaStreamer -- while this one was artwork.Service,
the only X.Service in the tree. It becomes artwork.Artwork/NewArtwork,
and serving.go follows the type to artwork.go.
ProvideImageStore was likewise the only "func Provide" in the repo; it
is GetImageStore now, matching GetImageCache beside it. cmd/wire_gen.go
regenerated via make wire.