does it for easy-install:

- require the download cache (but not namespace directories) to be an
  existing directory in order to catch configuration bugs

- made relative download cache paths refer to the buildout directory


git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@102730 62d5b8a3-27da-0310-9561-8e5933582275