monopam: track ocaml-sexpt rename to ocaml-sexp
Swap the [sexpt] dependency for [sexp] in dune-project, lib/dune, and
the opam file. Drop the [module Sexp = Sexpt.Sexp] aliases in
dune_project.ml and lint.ml (the new package exports Sexp directly).
In dune_project.ml, [parse_string_many]'s error type changed from
[string] to a structured [Sexp.Error.t]; format it explicitly with
[Sexp.Error.to_string] so the error message carries through to the
existing [err_parse] helper.