Missing VULCOIDS replacement implementation

[Hritik14]

The function to check if there is an existing VULCOID for an advisory with an alloted CVE is missing. We need to implement that i.e. effectively move VULCOID to from vulnerability_id to old_vulnerability_id.
Further we need to decide where the VULCOID should be generated in the importer-improver design.

Via: https://github.com/nexB/vulnerablecode/wiki/WeeklyMeetings#meeting-on-thursday-2021-09-14-at-1400-utc

[pombredanne]

We have changed the design and now a VULCOID is always assigned together with one to many to Aliases (such as a CVE id). Fixed in #525