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

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.

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

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.

The rounding algorithm wasn't specified

Serializing as ISO8601 makes the hash brittle, because the database may
change the timezone silently and/or lose precision in the microseconds.

As we do not need precise timestamp, using an integer is good enough,
and is consistant with the git format.

The manifest also does not need to contain a timezone, as it only
represents the timezone of the system that fetched this metadata,
which is useless data.

So that they can be properly deduplicated and referenced.

This will be used to compute an intrisic identifier for RawExtrinsicMetadata;
which can be used for deduplication and refering to it like any other sha1_git
instead of needed to use a tuple of its fields.

Other packages don't use it anymore.

This is a useful shorthand when generating SWHIDs in tests.

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.

* Quote/unquote path
* Fix line parsing and serializing to properly handle None
* Fix error raised by check_visit/check_anchor

They were all very similar and only differ in what 'edge' cases they accept

by making them all derive from an abstract class.

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 last one.

It also removes "ori" as a valid object type for CoreSWHID and
QualifiedSWHID, as it now only belongs in ExtendedSWHID.

It is cleaner, avoids warnings, and will be needed when introducing
ExtendedSWHID in a future commit.

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`