install sourceable this_iguana.* files to libexec/, not bin/

[c-dilks]

This prevents certain shells from calling

source this_iguana.___

from anywhere, since they may prioritize the version in $PATH over .; this is unlikely, since the name 'iguana' is rather unique, but better to avoid the problem anyway.

See JeffersonLab/coatjava#303 for a more problematic case, since many repos use env.sh as the sourceable file name.