Don't import JSON imports when file has no JSON writing types.
Don't import JSON imports when file has no JSON writing types.

hodduc/go-capnproto2,zombiezen/go-capnproto,zombiezen/go-capnproto2,hodduc/go-capnproto2,zombiezen/go-capnproto,tpukep/caps,zombiezen/go-capnproto2