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

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.

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.

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.