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.

- fix the printers themselves after discharge was moved to the kernel

- change the test to make it more robust
  In addition to checking that there is no "Error|Unbound" in the
  output, ensure that the "go" function at the end of base_include
  is defined. If there are any errors in base_include it won't be defined.

This makes us find out that the test was silently failing in all CI
jobs except trunk+make. It failed to find the "include" file and so
failed with "could not find file include.", which we didn't detect.
To fix this:

- change automatically included paths in Coqinit.init_ocaml_path to be
  based on coqlib instead of coqroot. This way top_printers.ml and
  base_include can find the compiled ml objects.

- fix for dune: adapt the script to use include_dune when INSIDE_DUNE,
  add dependencies to test-suite/dune.

  The dependencies should be calculated automatically once Dune has
  better support for debug, or we implement proper support for test
  printers.

Co-authored-by: Emilio Jesus Gallego Arias <e+git@x80.org>

This test is active only when configure `is_a_released_version` is set
to true. In this case, to pass the test-suite, there must be no
unreleased changelog entries left, i.e. `doc/changelog/*/` must only
contain `00000-title.rst` files.

We may want to keep the make-based and Dune job, however the
make-based setup is tested by the INRIA workers so it may not be
needed.

In order for some test to run well, we always run in Dune with an
absolute path. The easiest way to get a portable absolute path is to
use OCaml itself so we introduce a small executable to do that.

While we are at it, we do some cleanup of the test-suite `dune` file,
in particular we remove useless comments, set `--no-buffer` so results
can be seen in real time, and recognize the `NJOBS` variable as we
have moved to a Dune version that supports env vars.

We add a job testing the build of Coq with OCaml 4.08 [AKA `trunk`]

CoqIDE is not supported in 4.08 due to missing `lablgtk`, also `oUnit`
cannot be currently installed, thus we have to add a switch to the
test suite to disable `unit-tests`.

Many deprecation warnings happened in 4.08 so we use the `release`
profile to make them not fatal. Using a 4.08 build profile would be an
option too.