* Pass results, quiet and pgo to Logger constructor.
* Move display_progress() method from Regrtest to Logger.
* No longer pass Regrtest to RunWorkers, but logger and results.
* Add single.py and result.py files.
* Rename runtest.py to runtests.py.
* Move run_single_test() function and its helper functions to
single.py.
* Move remove_testfn(), abs_module_name() and normalize_test_name()
to utils.py.
* Move setup_support() to setup.py.
* Move type hints like TestName to utils.py.
* Rename runtest.py to runtests.py.