Implement a SiteLookup service based on JSON files, as proposed by T113034: RFC: Overhaul Interwiki map, unify with Sites and WikiMap.
The JSON structure should allow for all features we have in Site and in Interwiki. In addition, the concept of site groups should be extended to allow a single site to be in multiple groups.
When ingesting the JSON files, data from different files is deep-merged. For some fields, separate lookup arrays (indexes) are maintained: lookup by global id, group, by interwiki prefix, etc.