Fix creating stanfit from csv when rstan not loaded

[andrjohns]

Reported by @ave, using cmdstanr backend is giving error in some cases:

Error in getClass(Class, where = topenv(parent.frame())) : 
  “cxxdso” is not a defined class

Which is caused by the CSV processing attempting to use a class from the rstan package when it's not loaded. This PR updates the class lookup to specify that it should be found in rstan

[paul-buerkner]

Thanks! Merging now :-)