server-reason-react

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This PR is the first attempt as using the multi-context support for libraries with the same name that is being added in Dune (see related issue ocaml/dune#10222.

The gist of the improvement is that we define two Dune contexts for each "universal" project: an OCaml native context (default) and a Melange context (melange). Then, all native exes and libraries have to be annotated with (= %{context_name} "default")) and all Melange emit and libraries have to be annotated with (= %{context_name} "melange")), so that they get only get built under the proper context.

This approach allows to:

• remove copy_files rules from libraries with shared code
• get rid of the multi-folder layout for shared libraries (all modules and dune definitions can be in a single folder)
• use Dune wrapped libraries

Besides this, it should potentially be possible to get the editor definitions from the Melange or native cmts by leveraging the (merlin) field in Dune contexts, but something else seems to be broken that I will have to be investigate.

For now draft PR until a version of Dune is eventually released with those improvements.

See related discussion: melange-re/melange#694

In the last commits, I removed the last usages of copy_files in the url library. For this, I had to change the base branch to the PR in #127 so that we're able to include both implementations of the URL module in the same file.

We could also remove the wrapped false now in the url library, as both native and melange versions share the name. But this change was adding a lot more noise to the diff so maybe can be done separately.

The only missing parts are:

• wait for Dune to make a release to include the multi-context support
• update docs to remove references to copy_files

Doesn't seem we'll be moving forward with multi-context: https://github.com/ocaml/dune/issues/10507#issuecomment-2189900682