Start adding filemtime to config objects.

The config file changing should also be involved in expiring build
files. Components in builds can be shuffled around without modifying the
component files. This situation should trigger build refreshes.
Make isFresh() return false when the config file has changed.

If the config file has changed since a build was created, builds should
not be considered fresh. Config changes can dramatically alter the
output files without the contents of any other file changing.