buildout.py: Optimize _install_and_load by using cache.

_install_and_load is slow, using cache saves time when there are many sections.

src/zc/buildout/buildout.py

@@ -1341,9 +1341,15 @@ class Buildout(DictMixin):
     def __len__(self):
         return len(self._raw)
 
-
+_install_and_load_cache = {}
 def _install_and_load(spec, group, entry, buildout):
     __doing__ = 'Loading recipe %r.', spec
+
+    try:
+      return _install_and_load_cache[(spec, group, entry)]
+    except KeyError:
+      pass
+
     try:
         req = pkg_resources.Requirement.parse(spec)
 
@@ -1370,8 +1376,10 @@ def _install_and_load(spec, group, entry, buildout):
                 )
 
         __doing__ = 'Loading %s recipe entry %s:%s.', group, spec, entry
-        return pkg_resources.load_entry_point(
+        result = pkg_resources.load_entry_point(
             req.project_name, group, entry)
+        _install_and_load_cache[(spec, group, entry)] = result
+        return result
 
     except Exception:
         v = sys.exc_info()[1]