Backport of 'Read CVE enrichment from a verbatim offline mirror' to 19.3

What does this MR do and why?

Backport of !253401 (merged) to 19-3-stable-ee.

Offline Package Metadata Database sync cannot read a verbatim mirror of the CVE enrichment export. That export nests its files one directory deeper than the offline connector looks, so the sync ingests nothing and logs nothing. With this change the connector looks inside the data type directory when it exists, so a verbatim mirror and a mirror flattened by hand both sync.

The commit cherry-picks cleanly, with no adaptations. On 19-3, MalwareOffline, which master no longer has, also inherits this file_prefix from Offline; every malware sync configuration carries a purl_type, so its prefix does not change.

Testing

In an RSpec run, the package metadata model, service, worker and library specs pass on 19-3-stable-ee with this commit applied.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the E2E Omnibus GitLab EE pipeline (triggered by thee2e:test-on-omnibus-ee job) has succeeded. If it has failed (or shows warnings), investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.