We also get rid of ploc.ml, now useless, relying a priori on more
robust code in lStream.ml for location reporting (see
e.g. parse_parsable in grammar.ml).

As of today Coq has the `CProfile` infrastructure disabled by default,
untested, and not easily accessible.

It was decided that `CProfile` should remain not user-accessible, and
only available thus by manual editing of Coq code to switch the flag
and manually instrument functions.

We thus remove all bitrotten dead code.

Before you ask, the Ltac2.Ltac1 module is voluntarily underdocumented.

Fixes #13996: missing Ltac1.to_ident.

As a bonus ltac2 can produce bullet suggestions.

Instead of taking a type and checking that the inferred type for the expression
is correct, we simply pick an optional constraint and return the type directly
in the callback. This prevents having to compute type conversion twice in the
special case of Ltac2 variable quotations.

This should be 1:1 equivalent to the previous code, we are just moving code
around.

ssreflect asks setoid rewrite to rewrite in goal
"forall_special_name_ : T, _other_name_"

Since this is a non dependent product, setoid rewrite converts that to
"impl T _other_name_" and must be taught to restore the special name
when unfolding impl.

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: SkySkimmer

Fixes #13755 .