code: def validate(self, messages):
          
          messages = self.validate_checksum(messages)
          messages = self.validate_optional_str_fields(messages)
          messages = self.validate_mandatory_str_fields(messages)
          messages = self.validate_files(messages)
          messages = self.validate_mandatory_fields(messages)
          messages = self.validate_optional_fields(messages)
  
          return messages
docs: Validate the package fields.
          Append user friendly error messages to the `messages` list.