committer=None happens on some malformed commits generated by old dgit
versions ; and it is possible for author=None to happen for the same reason.

these fields are computed attributes and may be removed from the backend
storage. Helps writing tests.

This reverts commit a0f54362.

This means this commit removes the 'offset' and 'negative_utc'
arguments.

It also removes the 'negative_utc' attribute (which is not used
anymore), but keeps an 'offset' property, which is an alias to
'offset_minutes()'. This is only to keep this commit readable;
the next commit will remove this alias.

This should slightly reduce the memory used by dir/rev/rel objects

A future commit will remove the 'offset' attribute, and rename
'offset_bytes' to 'offset'.

Modules using the `.offset` attribute will need to switch to this new
method to keep working.

This allows keeping compatibility with existing users of the TimestampWithTimezone constructor

'offset' becomes a property instead of an attribute and constructor argument.

This also removes both from the output of `.to_dict()`.

This is step 6 of https://forge.softwareheritage.org/T3752

This will break packages that still use the constructor directly,
ie. swh-storage and swh-loader-git (and tests of swh-loader-core
and swh-loader-svn)

1. Most objects do not need it so it's a waste of space
2. This means we just extend the existing format (some objects will
   have that key in their dict) instead of changing it (retroactively
   adding it to all objects)

This will be used to store the original manifest of 'weird' git objects,
when we cannot reasonably represent them otherwise.

It calls attr.validate() (which calls the validators), and recomputes
the hash of HashableObject instances.

A future commit will also make it check the raw_manifest attribute
when relevant

It's just simpler this way

For the sake of completeness (a future commit may depend on it).

For now it is filled from 'offset' and 'negative_utc', but it will replace them
in a future commit.

This is to simplify and add support for more 'weird' offsets we do not currently
support.

I don't know any instance of these, but there is no harm in checking them.

1. hashes are now repr()ed as `hash_to_bytes("1234...")` instead of b"\x12\x34..."`
2. SWHID objects are now repr()ed as `CoreSWHID.from_string('swh:1:...:1234...')`
   instead of `CoreSWHID(scheme='swh', version='1', object_type=..., object_id=b'\x12\x34')`
3. enums are now repr()ed as `MyEnum.NAME` instead of "<MyEnum.NAME: 'value'>`

Thanks to these three changes, using repr() on a model object now prints
a string that can be pasted directly in a `.py` file to write a new
test case.

The previous replaced attrs-strict's type validator with our own,
stricter and faster, validator.

However, the strictness can be a burden in other packages;
for example, swh-storage tests rely on it to insert dummy data that raises
exception when accessed, and it would be hard to do while using the exact
expected type.

This commit reverts the strict behavior, but keeps the performance
optimization, by always checking with type equality, but in case type
equality fails (which would raise an error before this commit), it gives
the value a 'second chance', by trying isinstance.

This means that, outside tests, isinstance should not be used at all,
or very rarely.

This reimplements attrs_strict.type_validator(), using type equality
instead of isinstance.

This makes my checksum validation script (that mostly just instantiates
model objects, computes a checksum, then discard) run twice as fast.

Since they are used by the vault for non-identifier-related stuff,
I think it makes sense to move them to a new module.

identifiers.py is now an empty shell, as all its features were moved to
other modules and it only contains reexports and backward-compat
functions. Therefore, it should be considered deprecated from now on.

Refactor identifiers & model to make *_git_object() functions work on model classes instead of dicts

Since we now use these classes everywhere, computing hashes required
using to_dict() just to compute identifiers, which can be a performance
bottleneck in code computing many checksums.

identifiers.py initially worked only on bare sha1_git.

I chose to add the SWHID classes in that module because
of the name, but the SWHID code didn't actually interact
with the other functions in the module, so it now feels
out of place to me.

We're about to have a Bazaar loader

This class is a mixin that only works with classes that define
this attribute, so it makes sense to declare it.

It also allows generic functions (that take a HashableObject
parameter) to access it without 'type: ignore'.

This allows distinguishing multiple potential versions of the mapping
between external objects and their counterparts archived in Software
Heritage, for instance when a loader has a backwards-incompatible change
that should result in objects being loaded again.

The field defaults to zero, in which case it's backwards-compatible with
the previous implementation in terms of identifier computation.

This truncation is already enshrined at the identifier level. Truncate
the object itself as well, to reduce the possibility multiple different
metadata objects with the same identifier.

All other hashable objects but ExtId have one.

It will be used by swh-deposit.

this object aims at being able to keep in the SWH Archive an
  SWHID <-> External object ID
map, e.g. to be able to keep track of Mercurial ids so the Mercurial
loader can be made more efficient.

Related to T2849.

was modifying the dict given as argument.

So that they can be properly deduplicated and referenced.

SWHID is deprecated; and CoreSWHID does not support qualifiers at all,
so RawExtrinsicMetadata no longer needs to check there are no qualifiers.

ExtendedSWHID can identify either a software artifact or an origin,
so we no longer need Union[SWHID, str].

Therefore, we no longer need the 'type' attribute, as it was only
used to tell when the target is a SWHID and when it's an origin URL.

It can be handy as a shortcut to build SWHID objects.