The handling of Engine.GetAuxiliaryDir is a bit of a mess. diskSideloadStorage, and SSTSnapshotStorage both use the aux dir for storage, but assume that the filesystem routines in os will work on them. This is not true for in-memory engines and it tests only accidentally work because the RocksDB MemEnv does not implement full filesystem semantics (a parent directory is auto-created when a file is created).

In addition to cleaning up the handling of aux-dir, we should separate out the Engine.*File routines to their own interface. The pebble/vfs.FS interface is a good one to model on, though I think we only need a subset of that interface. For example, FS.Lock and FS.List do not appear to be needed.