Use lower level API to calculate signature

If we use options.items() we visit all keys and not used options cannot
be detected.

Rebase instructions:
- squash with "Add referred parts' hash strings in
  __buildout_signature__, that invokes rebuild of a part
  when one of its (recursive) dependencies are modified"
- add a comment explaining why such a lower level API is used

src/zc/buildout/buildout.py

@@ -1523,8 +1523,9 @@ class Options(DictMixin):
             self.buildout._parts.append(name)
 
         m = md5()
-        for item in sorted(self.items()):
-            m.update(('%r\0%r\0' % item).encode())
+        for key in sorted(self.keys()):
+            value = self._data.get(key, self._cooked.get(key, self._raw.get(key)))
+            m.update(('%r\0%r\0' % (key, value)).encode())
         self.items_signature = '%s:%s' % (name, m.hexdigest())
 
     def _do_extend_raw(self, name, data, doing):