Why do we have two sysconfig modules again?

And most of the tools.

Patch by Emanual Barry, reviewed by me, Serhiy Storchaka, and
Martin Panter.

Issue #27960: Revert state to 675e20c38fdac6, backing out all changes by developed for Issue #12885.

Patch by Ville Skyttä.

Several test cases in distutils use PyPIRCCommandTestCase as
their base class and as a result of that the following tests
were ran more than once:

* test_server_registration
* test_server_empty_registration
* test_config_interpolation

This commit moves the infrastructure used by other tests
into a new BasePyPIRCCommandTestCase class.