Following the discussion on T3034, we decided to replace SWHID with
two or three classes:
* QualifiedSWHID to replace the existing SWHID (standard types + qualifiers)
* ExtendedSWHID for internal use in Software Heritage (extra types + no qualifiers)
* maybe CoreSWHID, for "core SWHID" only (standard types + 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`