Fix #153: buildout should cache working set environments

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Fixes #397.

tox will produce the coverage reports by default.

This fixes #392.

Previously under Python 3 (probably prior to 3.6, when dicts became
ordered by default) or under Python 2 with ``-R`` or
``PYTHONHASHSEED=random`` the results were undefined because dict
iteration order is undefined.

The value being sorted is complex and defined by
pkg_resources.Distribution:

```    @property
    def hashcmp(self):
        return (
            self.parsed_version,
            self.precedence,
            self.key,
            _remove_md5_fragment(self.location),
            self.py_version or '',
            self.platform or '',
        )

    def __hash__(self):
        return hash(self.hashcmp)

    def __lt__(self, other):
        return self.hashcmp < other.hashcmp
...
```

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verbose mode and select sections for annotate command

avoid --section flag

--verbose allows, among others, to find out which values are overridden
when extending cfg files

--section allows to focus on a specific section

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It's not present in older setuptools versions.

It seems not to be needed since `pkg_resources.Environment.add()` will
sort dists in each project_name according to their version anyway.

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That was broken because the docs are no longer in the long description.

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* Document UserError

* Additional resources.

* briefly explain extensions

Later, we'll probably discuss how to develop them, probably in a separate document.

* move old changes to HISTORY.rst

* Cut over to new docs!

* Moved the meta-recipe test over to the manual.

It was already written as documentation using manuel.

The `unpack_wheel` was blindly trusting the result of `wheel_to_egg()`
and possibly laying zipped eggs in `eggs-directory`.

Fix it to always unpack the result of `wheel_to_egg()` instead.

* Make the Buildout helper class useful for testing recipes.

* Move run_buildout_in_process to testing because recipe integration tests may find it useful.

* Added support for special implication syntax ``=>``

* added <part-dependencies> support