Add lint check to warn if `ocaml` is not a dependency

[shonfeder]

Most of the time ocaml is a required dependency that we want to have explicitly stated (but not for conf- packages -- see #420).

The simplest fix here is to add a lint warning when ocaml is not declared as a dependency.

A smarter approach may be to check the package content to determine whether it needs this (e.g., looking for the presence of .ml files or asking opam if its deps require ocaml).

[shonfeder]

Fixing this would involve, at least:

• Adding a new check to the opam_repo_publication_shecks
  • This should probably include some logic to not give false positives in case the package is a conf- package, which may include making the new linting check conditional on is_conf_package
• Adding a new linting error lint_error.ml
• Adding some tests to the lint.t tests.

[ivnhk]

@shonfeder I can tackle this one after #432 if you don't mind

[mikejeuga]

@ivnhk I've been looking at it for a couple days and has started to work on this.

[ivnhk]

Sure, np

[shonfeder]

Thanks to you both! I've assigned @mikejeuga, and, if you're up for it, maybe you can help with review @ivnhk ?