[feat] Allow installable DEVELOP_DIST in index

Allow DEVELOP_DIST distributions found on sys.path by package index
when they contain a setup.py or pyproject.toml, as a good heuristic
of whether they will be installable or not.

src/zc/buildout/easy_install.py

@@ -482,7 +482,8 @@ class Installer(object):
         def filter_precedence(dist):
             return (dist.precedence == WHL_DIST) == wheel and (
                 not source or dist.precedence == pkg_resources.SOURCE_DIST) and (
-                dist.precedence != pkg_resources.DEVELOP_DIST)
+                dist.precedence != pkg_resources.DEVELOP_DIST or
+                any(f in os.listdir(dist.location) for f in ('setup.py', 'pyproject.toml')))
 
         index = self._index
         if not networkcache_failed: