Use the generic ppx driver support of Dune

[ghost]

This feature is part of a larger work to remove the hard-coded knowledge of various ppx projects in Dune: ocaml/dune#576

Dune can link custom ppx drivers on demand. Currently, it hard-codes the fact that ocaml-migrate-parsetree and ppx_driver provide entry points for such drivers. This is not very satisfactory and it forces changes to the system to be synchronized between the various projects, which is painful.

Starting from Jbuilder 1.0+beta20, individual libraries will be able to declare themselves as ppx drivers and Dune will use this information to decide what driver to use and how to use it, i.e. what flags to pass it. One can declare that a library is a ppx driver by adding a ppx.driver field to the library stanza:

(ppx.driver
 ((main       Migrate_parsetree.Driver.run_main)
  (flags      (--dump-ast))
  (lint_flags (--null))))

We are doing this change in two steps: in Jbuilder 1.0+beta19 which is being released, the ppx.driver field is accepted and ignored. In Jbuilder 1.0+beta20, it will be used and Jbuilder will now longer have hard-coded knowledge of ocaml-migrate-parsetree and ppx_driver.

This PR adds the ppx.driver field to src/jbuild and bumps the version constraint on Dune to Jbuilder 1.0+beta19.

Additionally, in the new system ppx_driver will automatically be selected as soon as one uses a ppx_driver based rewriter. As a result we need ppx_driver to accept all the command line flags accepted by the ocaml-migrate-parsetree driver. To do that, we need to retreive the command line arguments that where registered with the omp driver by individual rewriters. This PR adds functions to do that.

[let-def]

That patch looks good. I will wait for jbuilder19 to be available in opam to pass the CI test.

[ghost]

We just released beta18.1 which accepts and ignores the field as well, I changed the constraint

[ghost]

CI's passing. @let-def I'm merging this and preparing a release, so that we can move forward with the dune PR