Previous attempt to fix possible corruption was reverted because it had severe
performance issues (both CPU & storage).

Also, updating a part does not put it anymore at the end of the list of
installed parts. It was breaking uninstall dependencies.

This reverts commit 54bbe6bf.

Conflicts:
	zc.recipe.egg_/setup.py
	zc.recipe.egg_/src/zc/recipe/egg/egg.py

Currently only zc.recipe.egg:custom supports setting environment
variables, and zc.recipe.egg:develop does not.

My motivation for allowing setting environment in :develop is
wendelin.core

    https://lab.nexedi.cn/nexedi/slapos/blob/b5faab3b/component/wendelin.core/buildout.cfg

There we have [wendelin.core] part which installs released egg from
pypi, and [wendelin.core-dev] part which installs wendelin.core from
its latest git version via zc.recipe.egg:develop .

The problem is, wendelin.core for setup.py to work, needs git available,
and with slapos we usually don't have git available on base system, so
we build it by our own and do something like

    [wendelin.core-dev]
    recipe = zc.recipe.egg:develop
    environment = wendelin.core-dev-env

    [wendelin.core-dev-env]
    # wendelin.core-dev needs git to build
    PATH = ${git:location}/bin:%(PATH)s

and the problem is environment does not currently work for
zc.recipe.egg:develop, and thus git is not found -> build fails.

~~~~

In order to support environment in :develop, we just move environment
setting/restoring bits from Custom to Base, and provide Base.install() which
uses this bits. Custom & Develop .install() becomes ._install() which gets
hooked into Base.install() .

I've tested the patch only manually, because currently automated tests are
broken in a lot of places for slapos.buildout and zc.recipe.egg .

/cc @kazuhiko, @Tyagov

- Support on the fly patches in zc.recipe.egg by ``EGGNAME-patches``,
  ``EGGNAME-patch-options``, ``EGGNAME-patch-binary`` (or
  ``patch-binary``) and ``EGGNAME-patch-revision`` options.
- Support on the fly patches in zc.recipe.egg:custom by ``patches``,
  ``patch-options``, ``patch-binary`` and ``patch-revision`` options.
  (options ``EGGNAME-*`` are also supported as well).

Merge branch 'always_use_build'

Merge branch 'include_postrelease_string_in_version'

This reduces the risk of leaving a corrupted .installed.cfg

Version string is now N.N.N.postN to follow http://legacy.python.org/dev/peps/pep-0440/ .
(Ideally it should be N.N.N+SlapOSNNN, but version string containing '+' sign does now work
with older setuptools, that can be still used at the very beginning of bootstrap process.)

Conflicts:
	.gitignore
	setup.py
	src/zc/buildout/buildout.py
	src/zc/buildout/download.py
	src/zc/buildout/easy_install.py
	src/zc/buildout/testing.py
	src/zc/buildout/tests.py

Add referred parts' hash strings in __buildout_signature__, that invokes rebuild of a part when one of its (recursive) dependencies are modified.

Also remove duplicates and sort entries in __buildout_signature__.

       buildout-2-compatible recipes weren't expressed correctly,
       leading to unintendional use of zc.recipe.egg-2.0.0a3.