From an OCaml library point of view.

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.

This is actually supported by Sphinx directly.

The convention in the dune build is to be silent except for warnings
and errors, so they don't go unnoticed.

We could have this controlled by a variable if needed (likely would
require some support from Dune?)

Solves part of #12194

Dependencies specified through an alias do not trigger a rebuild when
they are modified.  This is likely a Dune bug, but we still need to
fix this on our side as this is inconvenient.

Since version 1.0.0 of the sphinxcontrib-bibtex extension, parallel
building of the Sphinx documentation emits a warning (and thus makes
our warning-free build fail).

This change was already done in Makefile.doc as part of #11732.

Move existing entries.

Instead, if the coqlib is special, we set it explicitly in the command
line, as Dune does.

This is a continuation of #9523.

In Sphinx, we stop using -boot, and pass `-coqlib` through the
environment instead.

Co-authored-by: Gaëtan Gilbert <gaetan.gilbert@skyskimmer.net>