(#20582) Restore facter_dot_d to stdlib for PE users

Without this patch Puppet Enterprise users who install the most recent
version of stdlib lose the ability to resolve certain facts critical to
the operation of Puppet Enterprise.  These facts are defined externally
in the file
`/etc/puppetlabs/facter/facts.d/puppet_enterprise_installer.txt`.  As an
example, Puppet Enterprise catalogs fail to compile if the
`fact_stomp_server`, and `fact_stomp_port` facts are not defined.

`facter_dot_d` was removed from stdlib version 4 because Facter version
1.7 now supports external facts defined in
`/etc/puppetlabs/facter/facts.d/puppet_enterprise_installer.txt`.
Puppet Enterprise does not yet include Facter 1.7, however.  The most
recent PE release, 2.8.1, includes Facter 1.6.17.  With this version of
Facter, users who replace the version of stdlib that ships with PE with
the most recent version from the Forge will lose the ability to resolve
facts from
`/etc/puppetlabs/facter/facts.d/puppet_enterprise_installer.txt`.

This patch addresses the problem by detecting if Facter version < 1.7 is
loaded.  If so, then the facter_dot_d.rb facts will be defined using the
stdlib custom fact.  If Facter >= 1.7 is being used then stdlib will not
define external facts.