SlapOS: don't fetch setup_requires eggs

Instead of installing setup_requires, which escapes buildout versions
constraints and allow-picked-versions, we disallow setup_requires to
install any missing packages. The packages are supposed to have been
installed earlier, by using setup-eggs= option of zc.recipe.egg:custom
or other variants of this recipe.

setuptools/__init__.py

@@ -10,6 +10,8 @@ from distutils.errors import DistutilsOptionError
 from distutils.util import convert_path
 from fnmatch import fnmatchcase
 
+import pkg_resources
+
 from ._deprecation_warning import SetuptoolsDeprecationWarning
 
 from setuptools.extern.six import PY3, string_types
@@ -136,7 +138,10 @@ def _install_setup_requires(attrs):
     # Honor setup.cfg's options.
     dist.parse_config_files(ignore_option_errors=True)
     if dist.setup_requires:
-        dist.fetch_build_eggs(dist.setup_requires)
+        # XXX SlapOS patch:
+        # we expect setup_requires to be already installed at this point
+        # instead of installing them, we throw an error if they are missing.
+        pkg_resources.WorkingSet().require(dist.setup_requires)
 
 
 def setup(**attrs):