For consistency, as the classes are now in swhids.py

+ raise warnings

1. Add a warning
2. Move identifier/manifest documentation to git_objects.py
3. Remove all imports of that module.

Motivation:

* SWHID classes were moved to swhids.py
* manifest computation functions were moved to git_objects.py
* Only reexports and trivial wrappers of model.py remain

They are not used anywhere.

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.

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.