setting to_scan to [] instead of None ensures (somewhat confusingly) that...

setting to_scan to [] instead of None ensures (somewhat confusingly) that find_links are used, when available, to install each dist listed in setup_requires

--HG--
branch : distribute
extra : rebase_source : 4efd4ba05e527df60a33c55da30c19586a9cecdb

setuptools/dist.py

@@ -266,6 +266,7 @@ class Distribution(_Distribution):
         """Fetch an egg needed for building"""
         try:
             cmd = self._egg_fetcher
+            cmd.package_index.to_scan = []
         except AttributeError:
             from setuptools.command.easy_install import easy_install
             dist = self.__class__({'script_args':['easy_install']})