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 D8539 about splitting some code to common module

Related swh-scanner!64 (closed)

Related swh-scanner#4591

Replaces !269 (closed)

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changed the description

mentioned in merge request !269 (closed)

Looks like a duplicate of !329 (closed).

@anlambert yep did rebase on a new branch. This one can be closed.

closed