And store their parsed values (CoreSWHID, tuple of ints, etc.) instead of string.

Following the discussion on T3034, we decided to replace SWHID with
two or three classes:

* QualifiedSWHID to replace the existing SWHID (standard types + qualifiers)
* CoreSWHID, for "core SWHID" only (standard types + no qualifiers)
* ExtendedSWHID for internal use in Software Heritage (extra types + no qualifiers)

This commit adds the second one

Following the discussion on T3034, we decided to replace SWHID with
two or three classes:

* QualifiedSWHID to replace the existing SWHID (standard types + qualifiers)
* CoreSWHID, for "core SWHID" only (standard types + no qualifiers)
* ExtendedSWHID for internal use in Software Heritage (extra types + no qualifiers)

Since migrating from SWHID will break existing code, this commit uses
the opportunity to modernize it a little, ie.:

* `keyword`-only constructor, to get rid of the hacky default values for
  `object_type` and `object_id`
* enum instead of strings for the object type
* `bytes` instead of an hex string for the object id
* rename `metadata` to `qualifiers`

Depending on the order in which tests are run, these tests may insert
lambdas with mocked blake2 functions in their closure to be inserted in
hashutil._blake2_hash_cache; causing all future tests to fail.

While this does not happen with the default order of tests, it does when
using pytest-xdist.

so it's kept up to date when evolutions are made in the model and thus
preventing swh-journal and swh-model to be unecessarly coupled.

Related to T2970.

Related to T2961

The optional nature of the type will allow to avoid migrating the
(db) data model right now then we can have this type field in kafka
messages in the origin-visit-status topic

Related to T2443

They were mixed in with snapshot tests.

test_identifiers: Make sure that {directory,revision,release,snapshot}_identifier() doesn't just return a value from the dict.

For example, before this commit, you could replace the code of
revision_identifier() with this:

def release_identifier(release):
    return release.get("id", b"")

and all tests would still pass.

Before this change there was a lot of overlap between parse_swhid() and the
attrs-based validators in the SWHID class. Also, the validation implementation
in parse_swhid() was done by hand.

With this change the coarse-grained validation done by parse_swhid() is now
delegated to a regex. The semantic validation of SWHIDs is left to attrs
validators. The regex is also exposed as a module attribute, to be used by
client code that want to syntactically validate SWHIDs without necessarily
instantiate SWHID classes (we have several other modules doing that already,
and they are using slightly different hand-made regexs, which isn't great).

As part of this change we also clean up the use of ValidationError exceptions,
systematically passing the problematic parts of SWHID as arguments, and uniform
error messages.

This change also brings some speed up in SWHID parsing. On a benchmark parsing
~30 M valid SWHIDs, the previous implementation took ~3:06 minutes, the new one
~2:50 minutes, or a ~9% speedup.

Closes T2788

Unfortunately, sphinx (actually, autodoc) only picks up attributes if
they fall in any of these cases:

1. are enum variants
2. are in slots
3. are in __dict__
4. have an annotation
5. are found using its custom parser

(see get_object_members in sphinx/ext/autodoc/importer.py)

In theory, option 5 should work for us; unfortunately, autodoc only
asks the parser the list of members with a comment.
And it's not easy to adapt it to ask the parser for all members,
because said parser (sphinx/pycode/parser.py) does not return the class
qualname (aka. namespace) for members without comments.

So, as I don't want to change the interface of sphinx.pycode.parser,
this commit switches to relying on option 3, by adding __slots__ for
all attr classes.

Additionally, this might have some performance/memory improvement
(though I did not check) and will further avoid mutation of these
objects.

All reverse dependencies have been updated to avoid using it now, so it can now
be removed, paving the way to recycle it into an intrinsic identifier.

Related to T2729

So parse_swhid raises a ValidationError when that is detected.

Related to T2769

Related to T2769

Related to T2769

Related to T2769

This avoids throwing the deprecation warning when id and target are present and
have the same value, which makes a from_dict / to_dict round-trip throw no
deprecation warnings.

This backwards-compatible change prepares the transition to give
RawExtrinsicMetadata an `id` field that is computed intrinsically from its
contents (using the HashableObject mixin).

This will enable a gradual enhancement of the functions in identifiers.py to
take model objects directly, and return the bytes of the hash instead of an
hex representation.

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

Related to T2610

flake8 hook has been removed from https://github.com/pre-commit/pre-commit-hooks
so now use the one from https://gitlab.com/pycqa/flake8

move most import statements in functions.

Related to T2575.

pytest wastes a lot of time in .hypothesis and .git; this commit excludes them.

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.