This is copied from the namesake distutils command; there is no
automated test, so buildbots won’t call for my head this time, but it
should be okay as Python 3 users have tested the distutils command.
In dry-run mode, packaging commands should log the same info as in real
operation and should collect the same files in self.outputs, so that
users can run a command in verbose and dry-run mode to see exactly what
operations will be done in the real run.
There was already a test for this, but it was complicated and had a
subtle bug (custom command objects need to be put in dist.command_obj so
that other command objects may see them) that rendered it moot.