allowed_eggs_from_site_packages inserts site-packages in the wrong place

When a requirement is filled by an egg in site-packages, if allowed_eggs_from_site_packages is true for that egg, then the directory for site-packages is inserts in the middle of buildout_paths list in the addsitepackages function of site.py. That means that all the eggs inserted afterwards may be ignored by any another egg in site-packages.

The following diff fixes the problem for me. I was unable to run the test suite cleanly, so I don't know if it's completely sane:

=== modified file 'src/zc/buildout/easy_install.py'
--- src/zc/buildout/easy_install.py 2010-04-29 18:11:45 +0000
+++ src/zc/buildout/easy_install.py 2010-06-11 05:19:03 +0000
@@ -881,7 +881,8 @@
                         _log_requirement(ws, req)
                         for dist in self._get_dist(req,
                                                    ws, self._always_unzip):
- ws.add(dist)
+ if dist.location not in self._site_packages:
+ ws.add(dist)
                             self._maybe_add_setuptools(ws, dist)
             if dist not in req:
                 # Oops, the "best" so far conflicts with a dependency.