New makes a DirCache This is created with the true root ID and the root path.

SplitPath splits a path into directory, leaf Path shouldn't start or end with a / If there are no slashes then directory will be "" and leaf = path.

DirCache caches paths to directory IDs and vice versa.

DirCacher describes an interface for doing the low level directory work This should be implemented by the backend and will be called by the dircache package when appropriate.