Add support for the new -H <dir> argument in OCaml 5.2

[kit-ty-kate]

Would fix:

• Debugging build failures with implicit_transitive_deps false #2733
• ignore ppx runtime dependencies when implicit transitive deps = false #4963

ocaml/ocaml#12246 added support for the -H <dir> argument, which, similarly to -I <dir> adds the given directory to the list of directories the compiler will look into for interfaces. However contrary to -I, -H forbids explicit calls to the modules and instead only resolves their type.

Using this would fix a foundamental flaw in the (implicit_transitive_deps false) option which causes regular annoyance for users and opam-repository maintainers alike.

Dune should be able to detect which version of the OCaml compiler it’s using, and if it’s OCaml >= 5.2, then instead of not adding any of the implicit directories in (implicit_transitive_deps false) mode, it would instead use -H, the same way -I is used with (implicit_transitive_deps true)

[nojb]

I took a quick look. A clean way to approach this would be if the calculation of the transitive closure of a set of libraries (eg Lib.closure) could return a list (bool * lib) list indicating whether each lib is in the root set or reachable by transitiveness. Maybe this is easy, I haven't checked.

[voodoos]

Note: In Merlin we are adding a new H configuration directive to Merlin to support this feature in the upgrade to 5.2. When support is added for this feature in Dune we should remember to send the appropriate configuration to Merlin.