publish: Access "-extends"-referenced sections during __init__

Buildout expects all needed sections to have been accessed during the
sectoin initialisation pass. It does not like having them accessed during
the "install" pass, and emits a very cryptic error:
  While:
    Installing publish.
  Error: Missing option: publish-early:__buildout_signature__

slapos/recipe/publish.py

@@ -31,19 +31,22 @@ from slapos.recipe.librecipe import GenericSlapRecipe
 CONNECTION_PARAMETER_STRING = 'connection-'
 
 class Recipe(GenericSlapRecipe):
-  def _install(self):
-    publish_dict = {}
-    done = set()
+  def __init__(self, buildout, name, options):
+    super(Recipe, self).__init__(buildout, name, options)
+    # Tell buildout about the sections we will access during install.
+    self._extend_set = done = set()
     extends = [self.name]
     while extends:
-        name = extends.pop()
-        done.add(name)
-        for k, v in self.buildout[name].iteritems():
-          if k[:1] == '-':
-            if k == '-extends':
-              extends += set(v.split()) - done
-          elif k != 'recipe':
-            publish_dict[k] = v
+      name = extends.pop()
+      done.add(name)
+      extends += set(self.buildout[name].get('-extends', '').split()) - done
+
+  def _install(self):
+    publish_dict = {}
+    for name in self._extend_set:
+      for k, v in self.buildout[name].iteritems():
+        if k != 'recipe' and not k.startswith('-'):
+          publish_dict[k] = v
     self._setConnectionDict(publish_dict, self.options.get('-slave-reference'))
     return []