Make ppx deriving compatible with preprocessor type ppxlib extensions

[kit-ty-kate]

This is an attempt at fixing the last issue of ppx_deriving with ocaml-migrate-parsetree 2.0

The problem was that making ppx_deriving a "preprocesor type ppxlib extension", made it imcompatible with other "preprocessor type ppxlib extensions" such as optcomp and triggered errors in a handful of packages in opam-repository.

From what I understand this was done to enforce an order between ppxs, and when mentioning this issue the ppxlib devs told me to use the Ppxlib.Deriving API. However, I'm no expert in either ppxlib nor ppx_deriving and didn't manage to make it work in a reasonable time.

Here I'm using the cheap method of simply stop caring about orders, I've ran a test on every opam packages for OCaml 4.11 with this change and no issue was found. So, to me, this seems safe to do so, since from what I understand, the previous code using OMP1 did not care about order either.

What do you think? @thierry-martinez @gasche

[ghost]

This change seems reasonable to me, and the new code makes more sense to me. You could probably find a combination of ppx rewriters where this is an issue, but you seem to have done enough testing here.

[kit-ty-kate]

Thanks a lot for the review! Merging and tagging the release now 🎉