Introduce package and setup.py less sdist creation

[obestwalter]

from ISSUES.txt originally created by @hpk42

Example sections for tox itself::

[pkgdef] 
basename = pytest
description = virtualenv-based automation of test activities
authors = holger krekel <holger@merlinux.eu>
url = http://tox.testrun.org
entry_points = console_scripts: tox=tox:cmdline
requires = py
packages = 
preuploadenvs = sphinx 

classifiers=
    Development Status :: 6 - Mature
    Intended Audience :: Developers
    License :: OSI Approved :: MIT License
    Topic :: Software Development :: Testing
    Topic :: Software Development :: Libraries
    Topic :: Utilities

This would generate three different packages:

• the main one containing the app with the specified description, etc.
  It has a test-requires pointing to the test package,
  which classifiers
• the test package only containing the tests and setup.py
  depending on the main package and all requirements collected from the
  testenv
• the doc package containing generated html and txt files
  (to be installable via a setup.py as well?)

Here is what happens when tox is invoked now:

• version gets auto-incremented (in setup.py and $PACKAGE/init.py files)

• main setup.py generated, files copied, sdist generated

• test setup.py generated, files copied, sdist generated

• doc setup.py generated, doc environment run, files copied, sdist generated

• if --upload is specified, all packages are uploaded under
  their respective names:
  tox-VER
  tests-tox-VER
  docs-tox-VER

• tox --sync creates a test result file for the tests-tox-VER run
  and uploads it to testrun.org -- .tox/projectkeys contains a file that
  was created by visiting testrun.org and registering/logging in.

• download toxslave and execute it:

  toxslave --userkey=... [--project tox]

  which will query testrun.org for outstanding testruns
  [for the tox project], download packages, execute them
  and report back to the server

[gaborbernat]

Package creation generally falls under the build backend. According to PEP-517, this should be part of the build tool and configured by pyproject.toml. Therefore I would suggest making this a won't fix and close it in favor of #573.

The solution likely will be creating a builder that upon pip build generates all those meta packages.