listCaches doesn't handle subdirectories

[nsheff]

listCaches only lists the contents of the main cache directory. It could separate them out by subdir and list them.

[vreuter]

Wanted to add just for my own clarity that it does list the names of the subfolders themselves, just not the contents. I hadn't previously noticed that.

[vreuter]

If we introduce a filter to partition contents into caches and non-caches (e.g., .RData suffix), listCaches could be modified to also accept a filepath (i.e., list caches somewhere else, without needing to change the cache dir).

[vreuter]

Not sure about others, but I sometimes make cache folders somewhat low down on the directory tree, and then have one of those active and want to reach over and see what's in other subtree(s)