Storing the found dist in the 'best' cache at the end

Whatever the method we used to find it, cache the result. Not just in one of
the cases. And it makes the method more readable as it doesn't mix functionality

src/zc/buildout/easy_install.py

@@ -653,7 +653,7 @@ class Installer:
                 dist = ws.by_key.get(req.key)
             if dist is None:
                 try:
-                    dist = best[req.key] = env.best_match(req, ws)
+                    dist = env.best_match(req, ws)
                 except pkg_resources.VersionConflict as err:
                     raise VersionConflict(err, ws)
             if dist is None:
@@ -669,6 +669,8 @@ class Installer:
                 # Oops, the "best" so far conflicts with a dependency.
                 raise VersionConflict(
                     pkg_resources.VersionConflict(dist, req), ws)
+
+            best[req.key] = dist
             requirements.extend(dist.requires(req.extras)[::-1])
             processed[req] = True
         return ws