buildout.py: Optimize _compute_part_signatures. Cache recipe signature and...

buildout.py: Optimize _compute_part_signatures. Cache recipe signature and calculate once per recipe.

Signature calculation is slow, so using cache saves a lot of time when the same recipe is used many times.

src/zc/buildout/buildout.py

@@ -918,14 +918,24 @@ class Buildout(DictMixin):
                     "Unexpected entry, %r, in develop-eggs directory.", f)
 
     def _compute_part_signatures(self, parts):
+        # The same recipe may appear many times.
+        sig_cache= {}
+
         # Compute recipe signature and add to options
         for part in parts:
             options = self.get(part)
             if options is None:
                 options = self[part] = {}
             recipe, entry = _recipe(options)
+
+            try:
+                # Copy cached list, because sig.append is called later
+                sig = sig_cache[recipe][:]
+            except KeyError:
                 req = pkg_resources.Requirement.parse(recipe)
-            sig = sorted(set(_dists_sig(pkg_resources.working_set.resolve([req]))))
+                sig_result = sig_cache[recipe] = sorted(set(_dists_sig(pkg_resources.working_set.resolve([req]))))
+                sig = sig_result[:]
+
             for dependency in sorted(options.depends):
                 m = md5()
                 for item in sorted(self[dependency].items()):