(#22944) Don't execute external facts 19 times

Previously, facter would load every external fact many, many times. For
executable facts, e.g. powershell, this means executing the fact. For
text based facts, it means reading the file, and parsing its output. On
Darwin, it was common for facts to be executed 6 times; on Windows,
19 times.

The problem occurs whenever facter requires a file, e.g. `ec2.rb`, which
references another fact, e.g. `:macaddress` outside the body of any
resolution, and facter has not yet required `macaddress.rb`.

For example, the following will cause all external facts to be loaded:

    if Facter.value(:macaddress)
      Facter.add(:ec2) do
        ...
      end
    end

Note that this is triggered because macaddress comes after ec2.
Similarly, `blockdevices.rb` references the `:kernel` fact outside of
any resolution, so that's another round of external fact evaluation.

This commit keeps adds a boolean flag to ensure we only execute external
facts once.