Print an error message and return non-zero status for
non-existing or unreadable files. Unknown options produce a
warning and are otherwise ignored.
Fixes #14023

Dependencies for Caml files was removed in PR #11589, but some
parts of it survived in the man page.

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.

Reviewed-by: Zimmi48

There is not need for coqdep to ship an `ocamldep` replacement, in
particular:

- not used in the main build since a long time
- not tested
- not kept up to date with upstream

This allows for a significant reduction of `coqdep` code, including
some duplicated code from `ocamllibdep`.

`coq_makefile` now uses `ocamllibdep` to process `mllib/mlpack` files,
so it has then to be installed.

We also remove the residual `-slash` option.

We remove the `dumpgraph` option which was causing quite a bit of
duplication, we also clean up options marked as broken `-w/-D`