(Re-)Enable module execution for all plugins

It's very useful (at least in development) to be able to execute plugins as modules, i.e. via

python3 -m igwn_monitor.plugins.check_url ...

rather than having to import the module and manually run the main function:

python3 -c "import sys; from igwn_monitor.plugins import check_url; sys.exit(check_url.main())" ...

We should add the necessary module suffix to enable module execution for all plugins:

# module execution
if __name__ == "__main__":  # pragma: no cover
    sys.exit(main())