fix: odoc driver linking to stdlib in $prefix/lib64

[katrinafyi]

This does two related changes to fix #1477:

• Use Findlib's own logic for inferring stdlib directory
• Adjust odoc_driver's classify_libs logic to also accept lib64/ and also accept files appearing outside of a package subdirectory, as the ocaml stdlib files do.

I can test that it works with dune exec odoc_driver -- fmt.

---

Removing the explicitly provided arguments to findlib will be more reliable than hardcoding a subpath of the opam prefix.

As of about 2 years ago, ocamlfind is able to automatically infer the config path from opam environment variables: https://github.com/ocaml/ocamlfind/blob/be2335f14cc45b5551dea4278affdfb2718eb72e/src/findlib/findlib_config.mlp#L71-L77

Also, hardcoding the env_camllib can be risky because it's not portable or overridable. On openSUSE Tumbleweed, my stdlib was actually in {prefix}/lib64 rather than the lib/ocaml which was hardcoded.

Findlib also has logic to let the user override the paths using environment variables, if needed: https://github.com/ocaml/ocamlfind/blob/be2335f14cc45b5551dea4278affdfb2718eb72e/src/findlib/findlib.ml#L281-L289

[jonludlam]

Thanks! I just put 'no changelog required' on this PR, but thinking about it, it's fixing a real bug that you found and that others might too - in which case it'd be useful to create an issue then have a changelog referencing it. Could you do that please?

[Leonidas-from-XIV]

Looking at the commit that added it the code is only available in ocamlfind >= 1.9.8, so I think it requires some updates to the lower bounds in odoc.

[katrinafyi]

Thanks! Added and updated the changelog, let me know if that looks okay.

I do also still intend to file the issue, as Jon suggested. I just wanted to double-check some things first and I'm away from the relevant computer right now.

[katrinafyi]

@jonludlam I've filed the issue and incorporated another small change which was needed to fully fix the issue :) Let me know how it looks.