[build] Split stdlib to it's own opam package.

We introduce a new package structure for Coq:

- `coq-core`: Coq's OCaml tools code and plugins
- `coq-stdlib`: Coq's stdlib [.vo files]
- `coq`: meta-package that pulls `coq-{core,stdlib}`

This has several advantages, in particular it allows to install Coq
without the stdlib which is useful in several scenarios, it also open
the door towards a versioning of the stdlib at the package level.

The main user-visible change is that Coq's ML development files now
live in `$lib/coq-core`, for compatibility in the regular build we
install a symlink and support both setups for a while.

Note that plugin developers and even `coq_makefile` should actually
rely on `ocamlfind` to locate Coq's OCaml libs as to be more robust.

There is a transient state where we actually look for both
`$coqlib/plugins` and `$coqlib/../coq-core/plugins` as to support
the non-ocamlfind plus custom variables.

This will be much improved once #13617 is merged (which requires this
PR first), then, we will introduce a `coq.boot` library so finally
`coqdep`, `coqchk`, etc... can share the same path setup code.

IMHO the plan should work fine.

2 files changed, 7 insertions, 2 deletions

diff --git a/toplevel/coqloop.ml b/toplevel/coqloop.ml
index 6460378edc..4faecd2e62 100644
--- a/toplevel/coqloop.ml
+++ b/toplevel/coqloop.ml
@@ -480,6 +480,11 @@ let drop_args = ref None
 (* Initialises the Ocaml toplevel before launching it, so that it can
    find the "include" file in the *source* directory *)
 let init_ocaml_path ~coqlib =
+  let coqlib : string =
+    if Sys.file_exists (CPath.make [coqlib; "plugins"] :> string)
+    then coqlib
+    else (CPath.make [ coqlib ; ".."; "coq-core" ] :> string)
+  in
   let add_subdir dl = Mltop.add_ml_dir (Filename.concat coqlib dl) in
   List.iter add_subdir ("dev" :: Coq_config.all_src_dirs)
 
diff --git a/toplevel/dune b/toplevel/dune
index 98f4ba2edf..9d5a08dde7 100644
--- a/toplevel/dune
+++ b/toplevel/dune
@@ -1,9 +1,9 @@
 (library
  (name toplevel)
- (public_name coq.toplevel)
+ (public_name coq-core.toplevel)
  (synopsis "Coq's Interactive Shell [terminal-based]")
  (wrapped false)
- (libraries coq.stm))
+ (libraries coq-core.stm))
 ; Interp provides the `zarith` library to plugins, we could also use
 ; -linkall in the plugins file, to be discussed.