Buildout should use cache system for pypi index

[Cédric de Saint Martin]

Currently, while trying to deploy a buildout based service with pinned egg versions, if an egg is not longer available (specified version has been hidden in pypi, or egg has been removed from pypi), it will fail.

Proper way to do would be to have a more generic extension system for buildout that would allow to expose and extend core functionalities of buildout (on this specific case: extension changing the easy_install module to rely on external cache server for specific pypi index requests).

[Domen Kožar (ielectric+)]

Proper way to fix this is to generate a simple html page with links to required distributions and provide find-links configuration to buildout. Until PyPI has a policty that disallows removal of packages, this is the best approach.