Add new class QualifiedSWHID to replace SWHID, and deprecate the latter.

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