utils/generate-cve-report.py: Add tables of unversioned elements of git/archive sources

This extends the CVE report with additional tables for any unversioned elements in the manifest, for 'git' and 'archive' source types.

The tables can be used to identify buildstream elements which may need their cpe version information set manually in the element, as it cannot be determined from the source information.

The tables of unversioned elements are also populated with links to any vulnerabilities linked to a particular element. As no version information has been extracted, this list will contain all CVEs related to element.