It adds primitives for finding the unknown parts of disk contents efficiently.
It provides ArchiveDiscoveryInterface, BaseDiscoveryGraph and RandomDirSamplingDiscoveryGraph classes for discovery algorithms.
This is a follow up of comments made in swh-scanner!64 about splitting some code to common module
Related swh-scanner!64 Related swh-scanner#4591
Also related to !269 (comment 128820)