Fix cache redundancy and ensure lookupUrl returns a Map
Resolves a stale data bug in the YouTube MusicBrainz Importer that was caused by a redundant, local cache (_urlCache) which was not being invalidated.
Removing this redundant cache revealed a subsequent TypeError, as the importer expected a Map from _mbApi.lookupUrl, but the MusicBrainzAPI.js library returned a plain Object. This commit refactors the MusicBrainzAPI.js library to have its lookupUrl method return a Map to fix this underlying issue.
