Set OCAMLTOP_INCLUDE_PATH to %{toplevel}%?

[ghost]

Since 4.08.0, OCaml supports a new environment variable OCAMLTOP_INCLUDE_PATH to specify additional include directories for the toplevel. Setting this variable would allow scripts installed in %{toplevel}% to be available in the OCaml toplevel without the user manually adding %{toplevel}% to the include path in the toplevel or package authors manually installing toplevel scripts in %{lib_root}%/ocaml.

opam already sets OCAML_TOPLEVEL_PATH to %{toplevel}%, however this variable is only recognised by utop and not by the vanilla OCaml toplevel.

I propose to start setting the OCAMLTOP_INCLUDE_PATH variable in the ocaml package, and eventually stop setting the OCAML_TOPLEVEL_PATH one. Thoughts?

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[ghost]

This is a not a super important issue, but is quite simple and still seems worth doing to me.

[ghost]

@dra27 do you think this change would break anything? This seems like a simple to change to make and it would make the life of people who want to install toplevel scripts easier in the future.