[opti] Compute default zc.buildout version lazily

src/zc/buildout/buildout.py

@@ -474,16 +474,17 @@ class Buildout(DictMixin):
         else:
             versions = {}
         versions.update(
-            dict((k, SectionKey(v, 'DEFAULT_VALUE'))
+            dict((k, SectionKey(v(), 'DEFAULT_VALUE'))
+                 # Use lambdas to compute values only if needed
                  for (k, v) in (
                      # Prevent downgrading due to prefer-final:
                      ('zc.buildout',
-                      '>='+pkg_resources.working_set.find(
+                      lambda: '>='+pkg_resources.working_set.find(
                           pkg_resources.Requirement.parse('zc.buildout')
                           # Skip local part because ">=x.y.z+abc" is invalid
                           ).parsed_version.public),
                      # Use 2, even though not final
-                     ('zc.recipe.egg', '>=2.0.6'),
+                     ('zc.recipe.egg', lambda: '>=2.0.6'),
                      )
                  if k not in versions
                  ))