[fix] Allow only installable DEVELOP_DIST in index

The package index considers may yield develop dists found on its local
search_path - sys.path by default - in addition to its index_url.

This commit discards such dists when they do not contain a setup.py
nor a pyproject.toml, as a good heuristic of whether they will be
installable or not.

src/zc/buildout/easy_install.py

@@ -481,8 +481,11 @@ class Installer(object):
         dists = [dist for dist in index[requirement.project_name]
                  if ((dist in requirement)
                      and
-                     ((not source) or
-                      (dist.precedence == pkg_resources.SOURCE_DIST)
+                     (dist.precedence == pkg_resources.SOURCE_DIST if source
+                      else not (dist.precedence == pkg_resources.DEVELOP_DIST
+                                and {'setup.py', 'pyproject.toml'}.isdisjoint(
+                                    os.listdir(dist.location))
+                                )
                       )
                      )
                  ]