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

@@ -1333,9 +1333,16 @@ 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
+
+    key = spec, group, entry
+    try:
+        return _install_and_load_cache[key]
+    except KeyError:
+        pass
+
     try:
         req = pkg_resources.Requirement.parse(spec)
 
@@ -1362,8 +1369,9 @@ 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 = _install_and_load_cache[key] = pkg_resources.load_entry_point(
             req.project_name, group, entry)
+        return result
 
     except Exception:
         v = sys.exc_info()[1]