Enable to check package documentation can be built without producing
sphinx warnings.

The sphinx environment is designed to be used in continuous integration
in order to prevent breaking documentation build when committing changes.

The sphinx-dev environment is designed to be used inside a full swh
development environment.

Related to T3258

Now that the .status file behavior is consistent, we can safely rely on
it during unit tests.

Close T2830.

Since those entries are updated in the background, we invalidate the
cache on a regular basis.

Close T2864. This should also fix the second problem of T2830, namely
the fact that get_entries(by_date, offset) was not reliable because of
the offset supposing the entries were the same every time.

Since metadata and history already share the same database, it makes
sense to share the connection as well, to prevent unnecessary locking on
the db file.

Related to T2830.

Invalidate specific cached artifact using `rm cache/{SWHID}`

Fix T2889.

this is more consistent with the style used by other SWH modules,
and also avoids parsing errors by our dependency graph generator

Fix T2825.

Close T2775.

Closes T2841.

Force a cache update on origin visits when last fetched more than one
day ago.

Fixes T2838.

Related to T2771.

Add a new 'cache/' directory along 'archive/' and 'origins/' in order to
navigate previously cached artifacts and for a future use-case about
cache invalidation on specific objects.

Close T2863.

Fix T2842.

Fix T2851.

See details and discussion in T2771.

- ls archive/ is now always an empty result
- Merge the meta/ directory content into archive/

Fix T2840.

Fix T2839.