Consider extends from relative URL

src/zc/buildout/buildout.py

@@ -1389,6 +1389,8 @@ def _open(base, filename, seen, dl_options, override, downloaded):
     _dl_options = _unannotate_section(dl_options.copy())
     newest = _convert_bool('newest', _dl_options.get('newest', 'false'))
     fallback = newest and not (filename in downloaded)
+    if _isurl(base):
+      fallback = fallback and not (base + '/' + filename in downloaded)
     download = zc.buildout.download.Download(
         _dl_options, cache=_dl_options.get('extends-cache'),
         fallback=fallback, hash_name=True)