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.

A future commit will make identifier computation use the attrs classes,
which are strict about what they accept.

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.

We agreed a while ago they should be IRIs and not just URIs.

This will trigger crashes in swh.storage.cassandra, as
currently expects (wrongly) that origin urls are ASCII.

* empty fetcher name or version is not accepted by cassandra
  (and is nonsensical anyway)
* ditto for non-ASCII (and any non-printable is nonsensical)
* null bytes/chars are accepted by neither postgresql or cassandra

This is the only value we should use from now on;
and the value is ignored by swh-storage anyway.

It will be used by swh-web.

Closes T3393

- add typing annotation to avoid such error in the future

Fixes T3383

the problem was for datetime<epoch, the timestamp is negative, but since
it's a float that includes the microseconds, if both are true (< epoch
and microsecond > 0), then the computed (int) timestamp was off by one.

Add dedicated tests for this.

make sure the snapshot id in OriginVisitStatus refers to existing
Snapshot objects.

If run after py3-full, it overwrites the full coverage info, so most lines
are incorrectly reported as uncovered in Phabricator diffs.

The git_object is what will be actually useful to the vault.

It's also easier to test, because test_identifier.py has the
entire git_object in its test data.

Before this commit, manifests were only computed internally before
hashing, so they were not available to outside modules.

This makes testing the module very painful, because identifier
functions can only be tested by checking the hash; so test failures
did not show mismatches between the computed manifest and
the expected one.

Additionally, the 'git bare cooker' of the vault is likely to use
these as well, as it needs to format git objects in the same format.

There is a regression that breaks attr.evolve() when updating attributes
that contain an attr class; which we use (eg. for Person or TimestampWithTimezone).

v21.2.0 is expected to fix the issue, but won't be released immediately:
https://github.com/python-attrs/attrs/issues/804#issuecomment-833471190

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

and add a test to keep them correct.

According to the SWHID specification, it is not forbidden for a qualifier
value to contain a '=' character (for instance in origin URL).

So update parsing code to handle that special case.

Some ValidationError exceptions could not be serialized to string due
to these format errors.

Related to T3234

Otherwise, they will get an error asking them to install Click
(or Dulwich if Click installed and they use -t snapshot)

And show nice human-readable errors instead

Was added in be8f1a55

Some releases don't have authors and date fields, this case should be
checked in the tests.

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.