maintenance: explicitely declare direct dependencies

[psafont]

Previously a lot of libraries were used but not declared because a dependent library was declared pulled them.

To enable the explicit mode (implicit_transitive_deps false) needs to be added to dune-project. I believe enabling it requires a high bar of knowledge around libraries, so it's best to leave it disabled. For example, the error when not importing declaring dune-build-info when it's needed is indecipherable:

File "ocaml/gencert/.gencert.eobjs/build_info_data.ml-gen", line 1:
Error: Could not find the .cmi file for interface
       ocaml/gencert/.gencert.eobjs/build_info_data.ml-gen.

While dune does not warn of unused libraries, I've removed some unused libraries, like xapi-idl (because only xapi-log was being used), or threads, which is not needed anymore as dune assumes -mt and forces linking against threads.posixanyway. Read https://dune.readthedocs.io/en/stable/dune-files.html?highlight=threads#implicit-transitive-deps-1 and https://dune.readthedocs.io/en/stable/advanced-topics.html?highlight=threads#findlib-integration

For users of the gzip and zstd I've decided to reexport the symbols from the library xapi-compression. I saw little value in forcing users to declare both.

[lindig]

So now that dependencies are correct, does it not make sense to use (implicit_transitive_deps false) and be alerted when libraries are missing? Otherwise we will be back where we were quickly.

[psafont]

I recommend against that setting, this is because sadly the dune library that provides an ocaml module is often not obvious. While I can guess it by grepping ocamlfind output, this is exceedingly difficult for non-experts.

I understand that it will get out-of-date but I rather have a PR a year updating it until dune includes a helpful error message telling users library xxx provides module yyy, please add it to the list of dependencies in file zzz