Deprecating "libraries" in opam files

[emillon]

The "libraries" field is supported but does not seem that anything in the ecosystem is using it:

https://github.com/ocaml/opam/blob/4d9db690420e36d995e6a0afe18660bcb45bea49/src/format/opamFile.mli#L460-L461

Might be worth deprecating it in 2.1 or 2.2.

[dra27]

The plan here:

• Double-check the lack of use and add an opam lint warning (all versions)
• When the opam file format itself is updated, remove the field

The lint warning forms the "deprecation" stage, since this field has no interpretation at present.

[AltGr]

I think that was a planned feature in 1.0 (when it was not clear if ocamlfind would be replaced) but never got any use; it was already deprecated in 2.0 and I think we can remove it altogether since it has always been absent from the manual. After double-checking that it's absent from opam-repo of course.

The exact same is probably true for the syntax: field as well ?

[kit-ty-kate]

maybe we don’t want to remove that. See https://github.com/ocaml/dune/issues/8650 In practice for that purpose it’s the same as having an x-* field but maybe we want to use it instead so we can have a feature to install a library like opam install --lib findlib to install ocamlfind