Add new class CoreSWHID as an alternative to SWHID/QualifiedSWHID

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