Lint {build} deps more gracefully

[pqwy]

Presently, topkg knows that

let lint_deps_default_excludes =
  let exclude = ["ocamlfind"; "ocamlbuild"; "topkg"] in

can be mentioned in opam but not in _tags, so every additional build tool needs this choreography:

let opams = [Pkg.opam_file ~lint_deps_excluding:(Some [tool]) "opam"]

Removing all of the deps marked as {build} in opam from the intersection test with _tags would help.

[dbuenzli]

I wanted to solve this for the next release, but actually accessing the information is more involved than I thought. So I'm removing the next label as I want to release. @AltGr did however promise me to help me...