This collapses the shared logic between these two identifier computations into a
few more explicit steps:
 - generate data for the manifest (in either identifier computation);
 - format the manifest (in the new format_manifest function);
 - hash the manifest (in the new hash_manifest function).

This will enable reusing this logic for more object types, as well as stronger
typing for the manifest computation.

that returns a value suitable for unicity constraints.

Motivation:

* this is somewhat more of a model concern than a journal/kafka
  concern IMO
* this is one step toward adding support for non-model objects in
  KafkaJournalWriter

Implementation of the unique_key methods comes from
`swh.journal.serializers.object_key`.

I created one in the wrong directory and didn't see the existing one.

The addition of '-p no:pytest_swh_core' in pytest.ini is needed to
prevent pytest from loading the pytest_swh_core plugin which we do not
need here and which would require some more dependencies (e.g.
requests).

Use the new SWHID naming convention instead of SWH PID.

Related to T2610

move most import statements in functions.

Related to T2575.

We may unknowingly pass naive datetimes to the storage through them,
causing the underlying DB to assign them a timezone that might not match
the actual one.

It already happens in swh.model and swh.loader.package tests.

Same as for the field data, it helps for code not yet migrated to use model
object.

Related to T645

It can help in pytest's diffs

to fix a deprecationg warning.

D3560

As NamedTuple inherits from tuple, msgpack serializes it like a tuple,
which makes it indistinguishable from a tuple when deserializing,
which is an issue for the RPC API.

It allows easy conversion of Union[ImmutableDict, Dict] to ImmutableDict.

So they are truly immutable now.

This will allow swh-storage to have a signature for *_metadata_add
that is consistent with other *_add endpoints.

Add a new extra_headers attribute on Revision and use it for computing
the revision's id instead of extract it from the metadata field.

Only accept (bytes, bytes) as extra_header.

Add a post init hook to Revision to initialize this new attribute from
given metadata, if any, for bw compat.

Also amend the revision_d hyptothesis strategy to generate extra_headers.

When Software Heritage persistent identifiers were introduced, they were not
yet abbreviated as SWHIDs.

Now that abbreviation is growing adoption, rename some functions and types in
swh.model.identifiers for consistency:

  - PersistentId -> SWHID

  - persistent_identifier -> swhid

  - parse_persistent_identifier -> parse_swhid

Backward compatibility with previous naming is maintained but deprecation
warnings are introduced to encourage the use of the new names.

Numerous variables in swh.model codebase have also been renamed accordingly.

Also rework and improve documentation.

Related to T2310

this aims at preventing constant usage of isinstance() based dispatch
code when writing generic code handling model entities.

For example, the "object_type" argument of JournalWriter.write_addition() has
become superflous now we only pass model entities, etc.

This idea comes olasd's reading of mypy doc:

  https://mypy.readthedocs.io/en/latest/literal_types.html#tagged-unions

This comes with a refactoring of from_dict.DiskBackedContent to make
it *not* inherit from model.Content: object_type being Final, it cannot
be overloaded.

Related to T2310

Related to T2310

This is not used. This is broken storage wise (origin-visit-add does not deal
correctly with it and it so happens there is no test around it).

And finally, this will soon go away with T2310.

And adapt the revisions_d() strategy accordingly.

This can be None, according to the model.

for the sake of consistency.

thus in TimestampWithTimezone.from_dict(). This is needed to help consuming
existing (invalid) messages from kafka.

Warning: tests added in this revision do not cover the whole
normalize_timestamp() function.

This is a step forward having model objects, declared as frozen, immutable.

This requires attrs_strict >= 0.0.7.