The rationale is that
 - it is not always the cmxs that is necessary .e.g., rdbg uses .cma or .ml
 - it makes more sense to refer to the source code

This is necessary because I cannot Dynlink .cmxs files from a ocaml
toplevel. Hence, one way to load the algos from rdbg is to
#load .cma files. This option generates an ocaml file that
can be #used by automatically generated rdbg-session.ml files.

Test: add enable rule so that step always do something (equivalent, but more in the spirit of the model)