[horde] horde-prefs, CLI, "Horde_Autoloader PEAR package is unavailable"

[Adam Tauno Williams]

I have a Horde instance that was installed like -

pear config-create  \
   /srv/www/vhosts/horde /srv/www/vhosts/horde/pear.conf
/srv/www/vhosts/horde/pear/pear  \
   -c  /srv/www/vhosts/horde/pear.conf \
   channel-discover pear.horde.org
/srv/www/vhosts/horde/pear/pear \
   -c /srv/www/vhosts/horde/pear.conf  install horde/horde_role
/srv/www/vhosts/horde/pear/pear \
   -c  /srv/www/vhosts/horde/pear.conf   run-scripts horde/Horde_Role
/srv/www/vhosts/horde/pear/pear \
   -c  /srv/www/vhosts/horde/pear.conf  install horde/horde
...

Now I need to use the horde-prefs script to migrate user's preferences
from one backend to another.  But I'm stumped as to how to get this CLI
script to run:

 php -dinclude_dir=/srv/www/vhosts/horde/pear/php -f
pear/horde-prefs /tmp/prefs_config.php adam list

I've tried many variations of this incantation and it always fails with:

The "Horde_Autoloader" PEAR package is unavailable. Install it with
"pear install Horde_Autoloader" first!

This class *is* installed, but I can't make the script environment find
it.

/srv/www/vhosts/horde/pear/php/Horde/Autoloader.php