BUG: re-initialize the master working set

In some cases (specifically when pip imports this module in a virtualenv),
pkg_resources can already be imported, causing setuptools to load
entry_points from an older version. Here, we re-initialize the master
working set to fix the case where the entry points from an older
setuptools are loaded.

setuptools/build_meta.py

@@ -35,6 +35,8 @@ import contextlib
 import setuptools
 import distutils
 
+from pkg_resources import _initialize_master_working_set
+
 
 class SetupRequirementsError(BaseException):
     def __init__(self, specifiers):
@@ -64,6 +66,7 @@ class Distribution(setuptools.dist.Distribution):
 def _run_setup(setup_script='setup.py'):
     # Note that we can reuse our build directory between calls
     # Correctness comes first, then optimization later
+    _initialize_master_working_set()
     __file__ = setup_script
     f = getattr(tokenize, 'open', open)(__file__)
     code = f.read().replace('\\r\\n', '\\n')