Remove redundant __main__ blocks from packaging modules.

The one interface we commit to maintain is the run module
(a.k.a. the pysetup script).
diff --git a/Lib/packaging/depgraph.py b/Lib/packaging/depgraph.py
index 843aab4..d633b63 100644
--- a/Lib/packaging/depgraph.py
+++ b/Lib/packaging/depgraph.py
@@ -224,6 +224,7 @@
 
 
 def main():
+    # XXX move to run._graph
     from packaging.database import get_distributions
     tempout = StringIO()
     try:
@@ -267,7 +268,3 @@
     else:
         print('Supported option: -d [filename]')
         sys.exit(1)
-
-
-if __name__ == '__main__':
-    main()