The project has been reorganized so that it can sensibly be packaged
either with or without tests, in both sdist and wheel forms, and
command-line config data is now handled in a way that allows users to
set up custom sdist options. The code is a quick self-demonstration of
this: the default configuration includes tests, but an sdist can be
prepared without them (and indeed, the tests depend on this
functionality).

The reorganization also helps set up for separate `bbbb` and `bbbb-dev`
packages. The "core" package will *dynamically* get the full package as
a dependency only at the moment when it's asked to make an sdist. This
way, Pip only grabs the bare wheel-building essentials when installing a
user package from an sdist, but developers who want to use these tools
to make sdists can use the same `pyproject.toml` configuration in the
local source tree that the shipped sdist will have. (Generally, users
won't explicitly install `bbbb`, and would only explicitly install
`bbbb-dev` if they're contributing to this project.)

Testing this setup properly will probably require setting up some demo
projects and building them without build isolation in specially
maintained venvs.

Currently there is still no proper way to customize how files are copied
from an sdist (or the original source tree) into a wheel; thus the
current tests for wheel builds fail, since they are expecting to see
test-less wheels. Nevertheless, this new functionality merits a version
bump, even if this version also won't be published on PyPI.