Handling missing DAG nodes

This is a long-standing and well-known issue, but I don't think a task was open about it yet.

When ingesting an origin, some nodes of the DAG may be missing, for various reasons:

• corrupted data (eg. a commit in the git history does not match its hash)
• directory must be found "somewhere else" (eg. SVN external (swh-loader-svn#611)
• revisions must be found "somewhere else" (eg. Bazaar stacked branches)
• ingestion of a (potentially large) repo might stop/crash after having ingested only some of its objects, and the repository might have disappeared when we try again

Currently, what happens is:

• if the missing object is a git object, then we know its sha1_git, and it's just a dangling reference (though this will be an issue when we will want to implement generation numbers, swh-storage#1617)
  • even in this (fortunate) case, other objects transitively referenced might remain completely unknown
• otherwise, objects referencing the missing object cannot even be represented in the SWH data model (and recursively, all objects referencing it)

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Migrated from T1957 (view on Phabricator)