Keep longer directory names

We want to be consistent with the previous layout, to avoid needlessly
breaking existing codes.  The header update means client codes are not
required to include all of the stepcode library includes explicitly -
i.e., stepcode itself does not force this requirement with its own
internal includes.  However, if client codes do not want to add the
library prefixes to their own includes as we have for stepcode, they
should still be able to add the various includes and work as before.
Adjust the include directory names accordingly.