Code owners - sort generated entries (elastic#198901)

## Summary

This PR adds sorting by path to the code owners generation. This
improves readability and also avoid problems where ownership entries for
nested test plugins (e.g.
`x-pack/test/alerting_api_integration/common/plugins/alerts
@elastic/response-ops`) are overridden with the empty top level test
package entry (`x-pack/test`) when it's placed further down in the code
owners file.