Avoid considering sys.path for distributions

When looking up a distribution to install an egg, keep strictly to
the provided index-url and find-links and do not use distributions
that are already on sys.path.

This change was introduced because of a situation where installing
an egg failed due to a bad distribution somehow available inside a
virtual environment. Could not remember how to reproduce.

src/zc/buildout/easy_install.py

@@ -149,7 +149,11 @@ def _get_index(index_url, find_links, allow_hosts=('*',)):
         index_url = default_index_url
     if index_url.startswith('file://'):
         index_url = index_url[7:]
-    index = AllowHostsPackageIndex(index_url, hosts=allow_hosts)
+    index = AllowHostsPackageIndex(
+            index_url,
+            hosts=allow_hosts,
+            search_path=[], # never look at sys.path
+            )
 
     if find_links:
         index.add_find_links(find_links)