buildout.py: Optimize _compute_part_signatures. Cache recipe signature and calculate once per recipe.

Signature calculation is slow, so using cache saves a lot of time when the same recipe is used many times.

This reverts commit 632b6015.

Since version 36, setuptools bundles again instead of having dependencies.

To support versions 33.1.1 and 36.6.1 of setuptools.

setuptools used to say this:

    No local packages or download links found for zc.buildout==UNKNOWN

Since setuptools 25.1.0 it says this:

    No local packages or working download links found for zc.buildout==UNKNOWN

See also https://github.com/pypa/setuptools/pull/609/files

(cherry picked from commit bcd3d9b5)

Contrary to bash, dash filters out environment variables
containing '-' characters.

Backporting an upstream changes, because  recently pypi stops accept download using http ( https://mail.python.org/pipermail/distutils-sig/2017-October/031712.html )

For slapos case, apparently it means all software releases where eggs where not in shacache stop working.

/reviewed-on nexedi/slapos.buildout!12

Another follow-up to fix MissingOption when the serialised value is None.

This fixes a syntax error on Python 3.

zc.buildout >= 2.5.3 is required to use newer versions.

This change is required to setup a test environment for this branch.

get is used by __getitem__, but also by other callers as it is public
method. So deserialize values if needed on each access by get, and
remove now needless deserialization in __getitem__

As Options.get is used internally, add Options._get without
deserialisation and use it in places, which expect clean result, and
adapt Options.get to be simple wrapper with deserialization without
using decoration with functools.

/reviewed-on !10

so that it can be imported even when buildout instance is not initialised.

Now specified patches are automatically applied on required eggs as well.

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

  While invoke setup.py certain eggs (like scikit-learn) launch
  cetain custom builds (for cython) using subprocess and sys.executable.

  This commit aims to preserve the sys.path over the runs, even if an
  egg is using subprocess with the same python to build a component of
  the egg.

   This commit is part of fixup the "Add setup-eggs option in zc.recipe.egg:develop."

This change fixes bug introduced in https://lab.nexedi.cn/nexedi/slapos.buildout/commit/aba00feb5cc694be6da433d936e29c94f9ae0416 for develop.

As _install_setup_eggs is part of Base class, some code
from Custom.__init__ had to be moved to Base.__init__ as those were
required by _install_setup_eggs implementation as self.link, options['_e'],
self.newest and so on.

Without this change zc.recipe.egg:develop fail to process.

Also, updating a part does not put it anymore at the end of the list of
installed parts, that was making .installed.cfg too big.