To speedup testing that module caches are up-to-date, the system
recognizes special file inside of each cluster. If .lastModified
file is present in a cluster, then the content of the cluster is not
scanned any more deeply and instead the time stamp of the file is
used as last modification for the whole content of the cluster.
This API is primarily intended for use by installer and RPM packagers
and also autoupdate, that are supposed to create such file, as soon
as they finish modifications to any cluster.
This file is ignored when accessing user directory, as it is expected
that user can modify it, while the user usually does not have access
permision or need to modify the shared installation clusters.