python: add qemu package installer

python: add qemu package installer

Add setup.cfg and setup.py, necessary for installing a package via
pip. Add a ReST document (PACKAGE.rst) explaining the basics of what
this package is for and who to contact for more information. This
document will be used as the landing page for the package on PyPI.

List the subpackages we intend to package by name instead of using
find_namespace because find_namespace will naively also packages tests,
things it finds in the dist/ folder, etc. I could not figure out how to
modify this behavior; adding allow/deny lists to setuptools kept
changing the packaged hierarchy. This works, roll with it.

I am not yet using a pyproject.toml style package manifest, because
"editable" installs are not defined (yet?) by PEP-517/518.

I consider editable installs crucial for development, though they have
(apparently) always been somewhat poorly defined.

Pip now (19.2 and later) now supports editable installs for projects
using pyproject.toml manifests, but might require the use of the
--no-use-pep517 flag, which somewhat defeats the point. Full support for
setup.py-less editable installs was not introduced until pip 21.1.1:
https://github.com/pypa/pip/pull/9547/commits/7a95720e796a5e56481c1cc20b6ce6249c50f357

For now, while the dust settles, stick with the de-facto
setup.py/setup.cfg combination supported by setuptools. It will be worth
re-evaluating this point again in the future when our supported build
platforms all ship a fairly modern pip.

Additional reading on this matter:

https://github.com/pypa/packaging-problems/issues/256
https://github.com/pypa/pip/issues/6334
https://github.com/pypa/pip/issues/6375
https://github.com/pypa/pip/issues/6434
https://github.com/pypa/pip/issues/6438

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Cleber Rosa <crosa@redhat.com>
Message-id: 20210527211715.394144-11-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>