adding new formatters can break older projects

In the ocaml-ci, we have a "lint fmt" step which probes the project metadata to determine the formatters to run over the project. This is primarily for dune files and ml[i] files via ocamlformat.

The problem arises when we try a dune build @fmt with ocamlformat installed but without a .ocamlformat file, at which point dune tries to invoke ocamlformat even if there is no way it can work:

File x.ml          
Warning: Ocamlformat disabled because [--enable-outside-detected-project] is not set and no [.ocamlformat] was found within the project (root: ../../../x)

I think there's an erroneous default in dune that makes this difficult to work around:

By default, formatting will be enabled for all languages and dialects present in the project that dune knows about.

This default means that projects have to specifically add (formatting disabled) to turn it off for OCaml code, but to enable it for dune files.

Wouldn't it be better to specifically version formatters used by a project, so that a future dune client understanding a new formatter wouldn't cause older format lint tests to fail? Specifically, this means flagging that ocamlformat is enabled specifically for a project. We could minimise breakage by taking the presence of an .ocamlformat file to mean (formatting enabled), and the absence to be (formatting disabled).

See https://github.com/ocurrent/ocaml-ci/issues/224