Stop abusing mr.developer

• Previously, an EDRN Portal egg would have a buildout that extended egg-
  base.cfg as well as its own setup.cfg.
• The setup.cfg would list "source dependencies"; that is, eggs needed by a
  package that we also happen to develop.
• We used mr.developer to automatically check those out.
• This got out of hand, especially in distributed environments which didn't have
  access to the source repository.
• Now we just let buildout grab dependencies normally.  Sure, they're not the
  latest release, but this software has stabilized enough now so that's OK.