Additional mapping: bibxml4 source data now available

[ronaldtse]

Given that:

• https://github.com/relaton/relaton-w3c/issues/24 is merged
• https://github.com/relaton/w3c-tr-archive is now available (the dataset of all historic W3C tr.rdf entries)

The historic W3C data is now available in https://github.com/relaton/relaton-data-w3c . (as well as the ietf-tools version of the repository)

We can now build a proper mapping for bibxml4 entries to the authoritative dataset from 2002 onwards.

If there is any data missing, it means the https://github.com/relaton/w3c-tr-archive repo is missing information.