Need a way to skip info from Specref for a spec

[tidoust]

Some entries in Specref are currently hardcoded in biblio.json but could rather be contributed by browser-specs, with more accurate information. There's no way to do that properly today because browser-specs leverages the info from Specref whenever it can. Some mechanism is needed to tell browser-specs not to fetch the information from Specref for some specs.

That's typically a temporary need, while the entry gets moved in Specref from biblio.json to browser-specs.json.