packages: avoid race in use of sys.modules

By iterating with sys.modules.iteritems(), we're subject to a race
condition: if there's another thread (e.g., in scons) that does an
import, sys.modules will change while we're iterating over it.

python/lsst/base/packages.py

@@ -91,7 +91,10 @@ def getPythonPackages():
             pass  # It's not available, so don't care
 
     packages = {"python": sys.version}
-    for name, module in sys.modules.iteritems():
+    # Not iterating with sys.modules.iteritems() because it's not atomic and subject to race conditions
+    moduleNames = sys.modules.keys()
+    for name in moduleNames:
+        module = sys.modules[name]
         try:
             ver = getVersionFromPythonModule(module)
         except: