[fix] Allow only installable DEVELOP_DIST in index

The package index considers may yield 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

@@ -481,7 +481,9 @@ class Installer(object):
         wheel = getattr(requirement, 'wheel', False)
         def filter_precedence(dist):
             return (dist.precedence == WHL_DIST) == wheel 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))))
 
         index = self._index
         if not networkcache_failed: