Refactor how files are watched

LoadedFile is now WatchedFile (since it has nothing to do with loading
files, only with telling when they change), and some of its internal
variables have been renamed for clarity. Also added a FileWatcher class
whose entire purpose is to track a set of LoadedFiles in aggregate. This
is now used by TypeCollection in place of its own hash.

Puppet::FileServing::Configuration::Parser and
Puppet::Util::NetworkDevice::Config were inheriting LoadedFile, simply
for its changed? method, even though neither of them really has file
properties. Now they wrap a WatchedFile instance instead.