[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, 14 insertions, 6 deletions

diff --git a/tools/coqdoc/dune b/tools/coqdoc/dune
index e3c792f277..cc888a959f 100644
--- a/tools/coqdoc/dune
+++ b/tools/coqdoc/dune
@@ -1,6 +1,6 @@
 (install
  (section lib)
- (package coq)
+ (package coq-core)
  (files
   (coqdoc.css as tools/coqdoc/coqdoc.css)
   (coqdoc.sty as tools/coqdoc/coqdoc.sty)))
@@ -8,7 +8,7 @@
 (executable
  (name main)
  (public_name coqdoc)
- (package coq)
- (libraries str coq.config coq.clib))
+ (package coq-core)
+ (libraries str coq-core.config coq-core.clib))
 
 (ocamllex cpretty)
diff --git a/tools/coqdoc/main.ml b/tools/coqdoc/main.ml
index b8d5032373..c95d1ee7db 100644
--- a/tools/coqdoc/main.ml
+++ b/tools/coqdoc/main.ml
@@ -471,9 +471,17 @@ let index_module = function
   | Latex_file _ -> ()
 
 let copy_style_file file =
-  let src =
-    List.fold_left
-      Filename.concat !Cdglobals.coqlib_path ["tools";"coqdoc";file] in
+  (* We give preference to coqlib in case it is overriden *)
+  let src_dir = CPath.choose_existing
+      [ CPath.make [ !Cdglobals.coqlib_path; "tools"; "coqdoc" ]
+      ; CPath.make [ !Cdglobals.coqlib_path; ".."; "coq-core"; "tools"; "coqdoc" ]
+      ] |> function
+    | None ->
+      eprintf "coqdoc: cannot find coqdoc style files\n";
+      exit 1
+    | Some f -> f
+  in
+  let src = (CPath.relative src_dir file :> string) in
   let dst = coqdoc_out file in
   if Sys.file_exists src then copy src dst
   else eprintf "Warning: file %s does not exist\n" src