[m-rev.] for review: start using the new code in file_names.m

[Julien Fischer]

On Sun, 11 Jun 2023, Julien Fischer wrote:

>>>  I will also check the effects of this change on the Opturion code base.
>
> I have sent you a program off-list that demonstrates two problems I
> found when attempting to compile G12.

In fact there's a much simpler test case for this: just try to compiler
the hello world in the samples directory as follows:

    $ mmc --use-grade-subdirs --make hello

Julien.