[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.

1 files changed, 12 insertions, 2 deletions

diff --git a/sysinit/coqloadpath.ml b/sysinit/coqloadpath.ml
index 8635345e00..95ae5da3de 100644
--- a/sysinit/coqloadpath.ml
+++ b/sysinit/coqloadpath.ml
@@ -44,8 +44,18 @@ let init_load_path ~coqlib =
   let coq_path = Names.DirPath.make [Libnames.coq_root] in
 
   (* ML includes *)
-  let plugins_dirs = System.all_subdirs ~unix_path:(coqlib/"plugins") |> List.map fst in
-
+  let unix_path =
+    (* Usually lib/coq-stdlib/../plugins ; this kind of hacks with the
+       ML path should go away once we use ocamlfind to load plugins *)
+    CPath.choose_existing
+      [ CPath.make [ coqlib ; "plugins" ]
+      ; CPath.make [ coqlib ; ".."; "coq-core"; "plugins" ]
+      ] |> function
+    | None ->
+      CErrors.user_err (Pp.str "Cannot find plugins directory")
+    | Some f -> (f :> string)
+  in
+  let plugins_dirs = System.all_subdirs ~unix_path |> List.map fst in
   let contrib_ml, contrib_vo = build_userlib_path ~unix_path:user_contrib in
 
   let misc_ml, misc_vo =