Make use of an egg cache to speed up install times

[pigeonflight]

The Plone unified installer provides a cache of some of the packages needed when running buildout. This reduces the installation time. The Plone heroku buildpack has a good example of "borrowing" the build cache of the unified installer to speed up installation see: https://github.com/plone/heroku-buildpack-plone/blob/9c115ebd9c0471ebab679f17a294880eb2f86b4d/bin/compile

A similar approach could be taken by plonecli.

Proposed implementation

A cli command called install_package_cache. When invoked it would:

1. download the unified installer to a temporary directory
2. unpack the installer into the temporary directory
3. unpack the cache into the target egg directory
4. clean up by removing the temporary directory

The ploncli build command would invoke install_package_cache as part of the build process.

[MrTango]

I use a global egg cache, otherwise you have to download every and fill the cache for every project.