cli: allow invoking the package

This allows `python -m dulwich` to work, consistent with e.g. `pip`.
`python -m dulwich.cli` still works as well.

dulwich/__main__.py

@@ -0,0 +1,4 @@
+from . import cli
+
+if __name__ == "__main__":
+    cli._main()

dulwich/cli.py

@@ -747,9 +747,13 @@ def main(argv=None):
     return cmd_kls().run(argv[1:])
 
 
-if __name__ == "__main__":
+def _main():
     if "DULWICH_PDB" in os.environ and getattr(signal, "SIGQUIT", None):
         signal.signal(signal.SIGQUIT, signal_quit)  # type: ignore
     signal.signal(signal.SIGINT, signal_int)
 
     sys.exit(main())
+
+
+if __name__ == "__main__":
+    _main()