Monitored variants of createDirectory and getDirectoryContents.
Define thee wrappers in the RebuildMonad module so we have fewer
open-coded tricky monitorFiles calls.

In particular replace a glob monitor on the content of the store with a
monitor on the store directory itself. This is valid based on the
behaviour of directory mtimes, which is specified by posix and we have a
sanity check for it in the unit tests.