Improve Vulnerability data in XLSX output

[mjherzog]

I ran the find_vulnerabilities add-on pipeline and then downloaded the XLSX output. The XLSX output had entries for the vulnerable packages in the affected_by_vulnerabilities field, but there are 2 significant problems:

• In some cases there is a message: "The value of: affected_by_vulnerabilities has been truncated from: nnnnn to 32767 length to fit in an XLSX cell maximum length". In these cases there is no data. The truncated data would be more useful with some indicator that it is truncated.
• The vulnerability information is one very long string - I could not figure out how to parse it in Excel with Text to Columns or similar.

The solution to both problems may be to create a new VULNERABILITIES sheet in the XLSX output. Vulnerabilities are a first-class data element so it seems reasonable to do this. The current affected_by_vulnerabilities field in the PACKAGES and DEPENDENCIES sheet could be repurposed as a yes/no flag so that the new VULNERABILITIES sheet could report PURLs from both the PACKAGES and DEPENDENCIES sheets.

[tdruez]

VULNERABILITIES sheet implemented in #1531