Add module-prefix optional argument to Ortac/QCheck-STM and Ortac/Dune

[n-osborne]

Fixes #268

This PR proposes to add a new optional argument to both Ortac/QCheck-STM and Ortac/Dune (the latter to use the former).
This new optional argument (-m or --module-prefix=) allows to test a module that is exposed as part of a library.

For example in:

(library
 (name myLib)
 (modules myModule))

You need to give myModule as the FILE argument, but it is not the right identifier for the module to open in the tests.

With this patch, you can run:

$ ortac qcheck-stm myModule.mli myConf.ml --module-prefix=MyLib

And the corresponding optional argument has been added to Ortac/Dune.

Beware that this is not fit to test sub-modules. I plan to cook a sister PR for that before the soon-to-come next release.

[n-osborne]

CI is green, I'm confident that the code is correct.
But if anyone looking around could take a look at the documentation update, it would be nice.

[jmid]

CI is green, I'm confident that the code is correct.
But if anyone looking around could take a look at the documentation update, it would be nice.

I had a look and the documentation LGTM (modulo a minor nit) 😃 👍

[n-osborne]

Thanks! Merging now.