Update the existing importers to use the reference type

[ziadhany]

issue: https://github.com/nexB/vulnerablecode/issues/1487 related pull request: https://github.com/nexB/vulnerablecode/pull/1518

The importer list that required an update:

• [ ] nvd,
• [ ] github
• [ ] gitlab
• [ ] npm
• [ ] pypa
• [ ] nginx
• [ ] pysec
• [ ] alpine_linux
• [ ] openssl
• [ ] redhat
• [ ] debian
• [ ] postgresql
• [ ] archlinux
• [ ] ubuntu
• [ ] debian_oval
• [ ] retiredotnet
• [ ] apache_httpd
• [ ] mozilla
• [ ] gentoo
• [ ] istio
• [ ] project_kb_msr2019
• [ ] suse_scores
• [ ] elixir_security
• [ ] apache_tomcat
• [ ] xen
• [ ] ubuntu_usn
• [ ] fireeye
• [ ] apache_kafka
• [ ] oss_fuzz
• [ ] ruby
• [ ] github_osv