there is an error.  I'd been trying to avoid writing a file, but that
made things too complicated.  I've seen some hard to reproduce test
failures that make turning on debugging in response to a failure
impractical.  Hopefully, if we get a test failure now, we'll be able
to see WTF.

In an attempt prevent extra logging in text output.  I'm not sure if
this results in less garbage output than we had before. It definately
reduces spurious output from the new getting-started test.

In any case, these should be cleaned up.

But getting failures when running with some other tests. Weird.

[ci skip]

[ci skip]

Fixes #326

Mainly to disable it when just trying to get distributions for
buildout and setuptools.

That I can no longer reproduce.

NFC

Fixed a bug introduced in 2.6.0:
zc.buildout and its dependeoncies were reported as picked even when
their versions were fixed in a ``versions`` section.  Worse, when the
``update-versions-file`` option was used, the ``versions`` section was
updated needlessly on every run.

[ci skip]

[ci skip]

Added a buildout option, ``abi-tag-eggs``

As suggested in: https://github.com/buildout/buildout/pull/325

Also simplified _setup_directories to not bother with the eggs
directory, since that's handled elsewhere.

that, when true, causes the `ABI tag
<https://www.python.org/dev/peps/pep-0425/#abi-tag>`_ for the buildout
environment to be added to the eggs directory name.

This is useful when switching Python implementations (e.g. CPython
vs PyPI or debug builds vs regular builds), especially when
environment differences aren't reflected in egg names.  It also has
the side benefit of making eggs directories smaller, because eggs
for different Python versions are in different directories.

Python 3.6