(#9546) Update configurer_spec for external facts pluginsync.

Prior to the #9546 work, there was only one download of plugins,
and now there may be two.  Update the configurer_spec to reflect
that change.
(#9546) Check that :pluginfactdest is a directory before using it

The #9546 work added the :pluginfactdest setting but didn't validate
that it was in fact a directory.  This adds that validation.  The
validation allows spec tests to pass, and it also covers the case
of someone mistakenly entering an incorrect path for this setting.
(#9546) Correct a typo in a setting desc.