support for hierarchical organization / boundaries

Well, #5 could provide a way for this to be implemented.
Use case -- git submodules/DataLad subdatasets. Caching should be done per each repository separately, even if they are in a hierarchy. So if we hit a "boundary" (e.g. submodule -- a directory with `.git` in it) we do not descend under assumption that that nested repository would have its own "fscached" call.