What is package base.caml?

I am trying to build parsexp-0.14.1 with caml-4.12 and it fails to find base.caml:

File "src/dune", line 1, characters 66-75:
1 | (library (name parsexp) (public_name parsexp) (libraries sexplib0 base.caml)
Error: Library "base.caml" not found.
Hint: try:
  dune external-lib-deps --missing @@default

opam doesn’t find base.caml. Based on its name it should be provides by ocaml itself (?) Where is this package defined?

base.caml is provided by Janestreet’s base library (see here). You probably need to run opam install base

Thank you!

Do you also know where does the package lwt.unix come from?

This is from the lwt package. You can install all dependencies of an opam package with

opam install . --deps-only