Currently we cannot block external facts. A mechanism similar to the one for internal facts does not work, because we already resolved the external facts at this stage, negating any performance effect of blocking.
We should implement blocking in directory_loader. Before calling the cache manager, we could block the facts (in this case they are represented by file names)
Allow users to block external facts. The blocking of external facts is a little different from the blocking of core and custom facts because we need to specify the name of the file from which external facts are loaded. Basically we can block files, not facts.
In order to block external facts, we need to specify the name of the file in the `blocklist` in `facter.conf`