Puppet apply does not add the stdlib lib directory to the $LOAD_PATH.
This is a problem because the puppet_vardir fact requires the
puppet_settings library to be available for the `with_puppet` utility
method.

Without this patch, puppet apply will result in the following error:

    $ puppet apply --modulepath=/vagrant/modules -e 'notice $puppet_vardir'
    warning: Could not load fact file stdlib/lib/facter/puppet_vardir.rb: no such file to load -- facter/util/puppet_settings
    notice: Scope(Class[main]):
    notice: Finished catalog run in 0.01 seconds

With this patch applied, puppet apply works as expected:

    $ puppet apply --modulepath=/vagrant/modules.pe -e 'notice $puppet_vardir'
    notice: Scope(Class[main]): /Users/jeff/.puppet/var
    notice: Finished catalog run in 0.01 seconds

This patch defensively tries to load facter/util/puppet_settings.  If it cannot
load it, it falls back to trying to explicitly locate and load the library.

Once puppet is fixed such that a modules lib directory is truly in the
$LOAD_PATH, the fall back implementation will no longer be exercised since the
LoadError should not be raised.