This should help avoiding issues like #13170.

Because of implicit relative imports, _backport.tarfile already imported
shutil from the backports instead of the stdlib; I have inlined the only
function it needed to remove the circular dependency between these
modules.

Logging calls have the signature (msg, *args, **kwargs) so that the
%-formatting can be delayed until it is needed.  Logger objects also have an
isEnabledFor method that can be used to isolate expensive code.

Next steps: use only one of d2.logger methods or logging module functions;
use a proper handler in our test machinery instead of monkey-patching; remove
cmd.warn and cmd.announce and use logging instead.  TODOs have been added in
the modules and on the wiki.