Changing the working directory for PPXs

[mrmr1993]

Projects using dune have their PPXs evaluated at the 'project root', some ancestor directory of the directory containing the current file. In the Coda project, we make use of this to load a global configuration file relative to the project root, using a fork of ppx_optcomp.

Currently, Merlin evaluates all PPXs in the same directory as the target. In our current setup, the ppx_optcomp fork then fails to find the file and aborts. This completely disables Merlin for that file.

It would be useful to have an option in .merlin to set the working directory for PPXs, so that the behaviour of PPXs under Merlin and dune are the same. Ideally, it would be good to coordinate with the dune project to add this option to its generated .merlin files.