test(melange): show work around for ocamldep issue + melange ppx

[anmonteiro]

This is an alternative version of #6625 with a workaround that's not manual.

Right now it's possible to overcome the limitation of ocamldep running before the melange PPX by:

• adding a (preprocess (action melc --as-ppx %{input-file})) field

This, however, has a pretty significant limitation: it isn't possible to run both actions + ppxes in a preprocess field (see #171).

So, it sounds to me that we need to solve this in melange too:

• expose a melange ppx library that works with the pps preprocessing stack
  • melange: move things around to prepare for melange.ppx library melange-re/melange#480 was already a step in this direction, by decoupling a few things in the melange source that make it possible to publish a separate PPX.

[jchavarri]

@anmonteiro I believe this can be closed now?

[jchavarri]

ref: #7323 (comment)

[anmonteiro]

Indeed this can be closed. We added tests in melange for this case.