[fix] zc.recipe.egg: Reinstall missing develop egg metadata

Let zc.recipe.egg:develop reinstall if .egg-info or .dist-info is
missing from the folder referenced by .egg-link.

zc.recipe.egg_/src/zc/recipe/egg/custom.py

@@ -167,9 +167,13 @@ class Develop(Base):
             options['setup'], options['_d'], self.build_ext)
 
     def update(self):
-        # If any option changes or the source directory is reinstalled
-        # or the .egg-link is deleted, install will be called instead.
-        pass
+        egg_link, = self.options.installed_files
+        with open(egg_link) as f:
+            path = f.readline().strip()
+        for e in os.listdir(path):
+            if e.endswith(('.egg-info', '.dist-info')):
+                return
+        return self.install()
 
 
 def build_ext(buildout, options):