text: Title: Change custom scalar imports
  Body: As discussed in #104
  
  If `parse`/`serialize`/`type`, in scalar section, consists `.` then produced import should be generated by splitting provided string by last `.`, eg. `"ab.cd.CustomScalar"` -> `from ab.cd import CustomScalar`
  
  If scalar section has `import` key then imports should be generated according to the existing strategy with raised deprecation warning