Access wideners should know their namespace

[Chocohead]

As mentioned on Discord, ever since FabricMC/fabric-loader@69f3edf, AccessWidenerReader#read doesn't need the currentNamespace and will ignore the access widener's if it isn't given. This seems unwise given if the active namespace genuinely isn't known, there is also no way of knowing whether the access widener being read is actually useful.

In Loader's case the access widener being read is remapped using a remapper mapping from intermediary mappings anyway, so it was known and AccessWidener never needed to be changed to allow null.