Factor out method which returns the final executable

[kaste]

Currently build_cmd and in turn get_cmd returns the complete computed cmd. It is useful though to just get the currently configured executable of a linter (type: str or list) so that a user can use our config system to query e.g. the version of such an executable.

This method could be named which_executable(cmd) -> str|list.

This method could be called get_executable() None -> Optional[List[str]]