[Distutils] mr.developer: share checkouts between buildouts

[Ross Patterson]

I often end up doing development on the same distribution for multiple
projects and have long wished for a simple way to share the same
clone/checkout between these multiple buildouts.

What would you think about supporting a shared sources directory much in
the same vein of zc.buildout's use of eggs-directory, zc.recipe.cmmi's
use of download-cache, and plone.recipe.zope2install's use of
zope-directory all of which can be set in the users
~/.buildout/default.cfg?

If the option is set, we could do the clone/checkout into the shared
sources directory and then symlink them into sources-dir.

Thoughts?
Ross