goose: use marker functions instead of comments

To avoid making this CL too large, I did not migrate the existing goose
comments through the repository.  This will be addressed in a subsequent
CL.

Reviewed-by: Tuo Shan <shantuo@google.com>

internal/goose/testdata/NamingWorstCase/foo/foo.go

@@ -17,8 +17,6 @@ func main() {
 	fmt.Println(c)
 }
 
-//goose:provide
-
 func provide(ctx stdcontext.Context) (context, error) {
 	return context{}, nil
 }