[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, 6 insertions, 3 deletions

diff --git a/lib/dune b/lib/dune
index 83783f9b5c..af30c9ae1f 100644
--- a/lib/dune
+++ b/lib/dune
@@ -1,7 +1,7 @@
 (library
  (name lib)
  (synopsis "Coq's Utility Library [coq-specific]")
- (public_name coq.lib)
+ (public_name coq-core.lib)
  (wrapped false)
  (modules_without_implementation xml_datatype)
- (libraries coq.clib coq.config))
+ (libraries coq-core.clib coq-core.config))
diff --git a/lib/envars.ml b/lib/envars.ml
index 1702b5d7a2..823d255f58 100644
--- a/lib/envars.ml
+++ b/lib/envars.ml
@@ -132,7 +132,9 @@ let guess_coqlib fail =
       if not Coq_config.local && Sys.file_exists (Coq_config.coqlib / prelude)
       then Coq_config.coqlib
       else
-        fail "cannot guess a path for Coq libraries; please use -coqlib option")
+        fail "cannot guess a path for Coq libraries; please use -coqlib option \
+              or ensure you have installed the package contaning Coq's stdlib (coq-stdlib in OPAM) \
+              If you intend to use Coq without a standard library, the -boot -noinit options must be used.")
   )
 
 let coqlib : string option ref = ref None
@@ -205,6 +207,7 @@ let print_config ?(prefix_var_name="") f coq_src_subdirs =
   let open Printf in
   fprintf f "%sLOCAL=%s\n" prefix_var_name (if Coq_config.local then "1" else "0");
   fprintf f "%sCOQLIB=%s/\n" prefix_var_name (coqlib ());
+  fprintf f "%sCOQCORELIB=%s/\n" prefix_var_name (if Coq_config.local then coqlib () else coqlib () / "../coq-core/");
   fprintf f "%sDOCDIR=%s/\n" prefix_var_name (docdir ());
   fprintf f "%sOCAMLFIND=%s\n" prefix_var_name (ocamlfind ());
   fprintf f "%sCAMLFLAGS=%s\n" prefix_var_name Coq_config.caml_flags;