distributions available. This required working around issues with
easy_install's --upgrade option:

- Upgrade is not recursive. Upgrading a distro doesn't update it's
  dependencies.

- Upgrade doesn't try very hard to avoid searching.  If we require a
  specific version of a distribution, and we already have that
  distribution, there's no point in looking for a newer one.

- easy_install has kind of odd rules for deciding when to look at an
  index.   Now that we use upgrade all the time, easy_install always
  wants to look at an index.

- We get warnings when connecting to index servers, like PyPI that 
  return text/plain not found messages.

We now have much greater control over how dependencies are
managed. We've essentially taken this over from easy_install.

Because we now always talk to an index server and because we want to
control anything we do in a test, many of the tests actually run their
own web servers.

Anyway:

- Now handle upgrades correctly, I think.

- The egg recipe can now install multiple distributions.

- We have the beginnings of offline mode.

- The internal architeture is much cleaner.

- We've merged the easy_install and egglinker modules, tossing
  some superfluois apis in the egglinker module.


git-svn-id: http://svn.zope.org/repos/main/zc.buildout/trunk@68838 62d5b8a3-27da-0310-9561-8e5933582275