cachevfs: add a vfs dedicated to cache Most of the cache content lives in '.hg/cache/'. Moreover they are computed exclusively from data in the '.hg/store' directory. This creates issues with the share extension as the '.hg/store' directory is shared but the '.hg/cache' is not. On large repositories, this makes this prevent some usage of the share extension inefficient as some caches can take minutes to be recomputed. To improve the situation, we introduce a new 'cachevfs' that will be dedicated to cache reading and writing. In the next patches of this series, we'll migrate the 4 existing caches to it and update the share extension.