* buildout.cfg: Remove redundancy

Share the eggs definition between the the online and offline tests.

* Remove redundant code

Trying to insert `self._dest` in `self._path`  in `Installer.install()`
is unnecessary since:

 * it's already done in `__init__()`

 * nothing changes either `self._path` or `self._dest` during the
   lifetime of `Installer`

Remove `Installer._load_dist(self, dist)` since it has never been
used in the whole history of Buildout.

Finally, simplify the interface to `._get_dist()` and  `_call_easy_install()`:

All callers to `._get_dist()` add the resulting dist to the WorkingSet. No
reason to do it in `._get_dist()` or to pass the `ws` into
`._call_easy_install()`.

* Be stricter when adding setuptools as requirements

Only do it for namespace package dists that actually need it. I.e. those
that use `pkg_resources.declare_namespace()` instead of
`pkg_util.extend_path()` or PEP 420.

* easy_install: `eggs-directory` may contain more than eggs

Modify the `easy_install.Installer` class to locate not only eggs, but
anything that has a `.dist-info` inside `._dest` (i.e. the Buildout
`eggs-directory`).

This makes it easier for extensions like `buildout.wheel` to add non eggs
inside it: by installing a distribution inside a subdirectory of `._dest` as if
that subdirectory was `site-packages`, you can later locate that
distribution with the Installer.

Conversely, make sure any distribution found in a direct child of `.dest` is
treated as if it was an `egg` distribution (as opposed to a develop
distribution or a site-packages distribution).

All access to `Installer._path` and all access that modifies `._env` is now
done through `Installer` methods, so that Buildout extensions can install
subclasses of `Installer` that alter their behaviour.

* Refactor `_get_dist()` and `_call_easy_install()`

Move the actual invocation of `easy_install` to a module global function
`call_easy_install()`. Simplify its signature so it's simpler to
override in Buildout extensions.

Refactor `._get_dist()` and remove all unpack/install logic into
`_move_to_eggs_dir_and_compile()`, bypassing
`Installer._call_easy_install()`.

`._get_dist()` now calls directly `call_easy_install()` instead. But
only as a fallback to a dictionary lookup of filename extensions to
distribution unpacking methods like:

 - `.egg`: `unpack_egg()`
 - `.whl`: `unpack_wheel()`

This will make it easier for extensions to add support to new dist
formats.