Removed version-unconstrained ws.by_key.get(), this finds too many

non-matching versions.

Ignoring version conflicts for buildout and setuptools, this enables
buildout/setuptools upgrades/downgrades after a bootstrap.

src/zc/buildout/easy_install.py

@@ -650,13 +650,17 @@ class Installer:
             logger.debug("Processing requirement %s (constrained to %s)",
                          current_requirement, req)
             dist = best.get(req.key)
-            if dist is None:
-                # Find the best distribution and add it to the map.
-                dist = ws.by_key.get(req.key)
             if dist is None:
                 try:
                     dist = env.best_match(req, ws)
                 except pkg_resources.VersionConflict as err:
+                    if req.key in ['zc.buildout', 'setuptools']:
+                        # We're bootstrapping zc.buildout with a different
+                        # version than the one we specified in our versions
+                        # list. Ignore the error it and we'll grab the right
+                        # one below.
+                        dist = None
+                    else:
                         raise VersionConflict(err, ws)
             if dist is None:
                 if dest: