PURLDB: Collect and return Alpine package metadata on demand for a PURL

We should get the details from an Alpine project on demand.
We should also enrich a binary with actual source scans like discussed prior in related issues and PRs.

- [ ] ScanCode.io: Collect the distro of a container to create proper Alpine package PURL

See also:
- https://github.com/nexB/scancode-toolkit/issues/3726
- https://github.com/nexB/scancode.io/issues/194
- https://github.com/nexB/scancode.io/pull/272
- https://github.com/nexB/purldb/issues/307
- https://github.com/nexB/scancode-toolkit/issues/2541
- https://github.com/nexB/scancode-toolkit/issues/2543
- https://github.com/nexB/scancode-toolkit/issues/2061
- https://github.com/nexB/scancode-toolkit/issues/2766
- https://github.com/nexB/scancode-toolkit/issues/2774
- https://github.com/nexB/scancode-toolkit/issues/1803

Some relevant references:
- This is a possible repo: http://dl-cdn.alpinelinux.org/alpine/edge/community/
- A tool https://github.com/privazio/alpine_release_info
- Parsing the YAML index file may be the way, for instance:
- http://nl.alpinelinux.org/alpine/v3.6/releases/s390x/latest-releases.yaml
- https://github.com/knqyf263/apkindex-archive is an historical index archive