Don't run main() as a side effect of the import.

This adds an import-time guard to ensure that `main()` isn't run twice
when command is run through the install-time shim.

Opted to leave this script in nav.bin, since it has pre-main setup
that isn't in the imported module.

python/nav/bin/navtopology.py

@@ -5,4 +5,5 @@
 
 from nav.topology.detector import main
 
-main()
+if __name__ == '__main__':
+    main()