fix cmd-. shortcut's call to set-position

(so it no longer sets the range to a strange thing (in some cases))
Define the pre-installed teachpacks to be those teachpacks
that are in the same directory as the image teachpack (for
htdp and htdp/2e)

closes PR 13967

This isn't the best solution: it would be better if I could
get the directory containing any collection whose name is
teachpack/htdp, but that doesn't seem easy with the current
set of functions

also, bring down below 102 columns