Fix Dune warnings on pesy preprocessors config.

[jordwalke]

If you specify a preprocessor, now recent Dune will complain that the generated .merlin is not accurate (that's fine).
Solution: apply the same preprocessor config to the pesy-modules library as well.

Discussed Here

The warning makes sense in the general case, but it can be fixed by making sure that even the pesy-modules has the same ppx applied to it.

Currently if you have:

"library": {
  "imports": [
    "Console = require('console/lib')",
    "React = require('static-react')"
  ],
  "preprocess": ["pps", "static-react.reactjs_jsx_ppx"]
},

The generated Dune will be:

(library (name AppLibrary) (public_name app.library)
    (modules (:standard \ AppLibraryPesyModules))
    (libraries app.library.pesy-modules)
    (flags -open AppLibraryPesyModules)
    (preprocess (pps static-react.reactjs_jsx_ppx)))
(library (public_name app.library.pesy-modules)
    (name AppLibraryPesyModules) (modules AppLibraryPesyModules)
    (libraries console.lib static-react))
(rule
    (with-stdout-to AppLibraryPesyModules.re
        (run echo  "module Console = Console;\
                  \nmodule React = StaticReact;")))

And Dune complains because it needs everything in a directory to have the same ppx config.

The solution is easy it seems: pesy should just make sure to add the same (preprocess) spec to the pesy-modules segment as well.

[ulrikstrid]

I thought this was fixed here #83

[jordwalke]

Maybe my pesy is out of date!? Let me check.