Draft: Add 'discovery' model
It adds primitives for finding the unknown parts of disk contents efficiently. It provides ArchiveDiscoveryInterface and BaseDiscoveryGraph classes for discovery algorithms.
This is a follow up of comments made in swh-scanner!64 (closed) about splitting some code to common module
Related swh-scanner!64 (closed)
Related swh-scanner#4591
Migrated from D8937 (view on Phabricator)