Add discovery model

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 (closed) about splitting some code to common module

Related swh-scanner!64 (closed) Related swh-scanner#4591

Also related to !269 (comment 128820)