Separate data import and data inference/refinements

[Hritik14]

#467 now refines the upstream data and puts it in the database as a fact. An importer should only do one job - import.
The data obtained by the importers should not be tampered with and be as true as the upstream.
The heuristics like time travel, finding fix commits (thus finding vulnerable versions) etc should be implemented in a different module than importers.

revisit time travel. It should not be part of importer but something that is about data improvement, inference and refinements
and something that is about data improvement, inference and refinements should be a separate tool

via https://github.com/nexB/vulnerablecode/wiki/WeeklyMeetings#meeting-on-thursday-2021-07-15-at-1200-utc

[Hritik14]

Fixed by #525