[loadpath] Rework and simplify ML loadpath handling

This PR refactors the handling of ML loadpaths to get it closer to
what (as of 2020) the standard OCaml toolchain (ocamlfind, dune) does.

This is motivated as I am leaning toward letting the standard OCaml
machinery handle OCaml includes; this has several benefits [for
example plugins become regular OCaml libs] It will also help in
improving dependency handling in plugin dynload.

The main change is that "recursive" ML loadpaths are no longer
supported, so Coq's `-I` option becomes closer to OCaml's semantics.

We still allow `-Q` to extend the OCaml path recursively, but this may
become deprecated in the future if we decide to install the ML parts
of plugins in the standard OCaml location.

Due to this `Loadpath` still hooks into `Mltop`, but other than that
`.v` location handling is actually very close to become fully
independent of Coq [thus it can be used in other tools such coqdep,
the build system, etc...]

In terms of vernaculars the changes are:

- The `Add Rec ML Path` command has been removed,
- The `Add Loadpath "foo".` has been removed. We now require that the
  form with the explicit prefix `Add Loadpath "foo" as Prefix.` is used.

We did modify `fake_ide` as not to add a directory with the empty
`Prefix`, which was not used. This exposed some bugs in the
implementation of the document model, which relied on having an
initial sentence; we have workarounded them just by adding a dummy one
in the two relevant cases.

2 files changed, 9 insertions, 16 deletions

diff --git a/doc/changelog/11-infrastructure-and-dependencies/11618-loadpath+split_ml_handling.rst b/doc/changelog/11-infrastructure-and-dependencies/11618-loadpath+split_ml_handling.rst
new file mode 100644
index 0000000000..77fa556321
--- /dev/null
+++ b/doc/changelog/11-infrastructure-and-dependencies/11618-loadpath+split_ml_handling.rst
@@ -0,0 +1,9 @@
+- **Changed:**
+  Recursive OCaml loadpaths are not supported anymore; the command
+  ``Add Rec ML Path`` has been removed; :cmd:`Add ML Path` is now the
+  preferred one. We have also dropped support for the non-qualified
+  version of the :cmd:`Add LoadPath` command, that is to say,
+  the ``Add LoadPath dir`` version; now,
+  you must always specify a prefix now using ``Add Loadpath dir as Prefix.``
+  (`#11618 <https://github.com/coq/coq/pull/11618>`_,
+  by Emilio Jesus Gallego Arias).
diff --git a/doc/sphinx/proof-engine/vernacular-commands.rst b/doc/sphinx/proof-engine/vernacular-commands.rst
index a38c26c2b3..d1f3dcc309 100644
--- a/doc/sphinx/proof-engine/vernacular-commands.rst
+++ b/doc/sphinx/proof-engine/vernacular-commands.rst
@@ -745,11 +745,6 @@ the toplevel, and using them in source files is discouraged.
    :n:`-Q @string @dirpath`. It adds the physical directory string to the current
    |Coq| loadpath and maps it to the logical directory dirpath.
 
-   .. cmdv:: Add LoadPath @string
-
-      Performs as :n:`Add LoadPath @string @dirpath` but
-      for the empty directory path.
-
 
 .. cmd:: Add Rec LoadPath @string as @dirpath
 
@@ -757,11 +752,6 @@ the toplevel, and using them in source files is discouraged.
    :n:`-R @string @dirpath`. It adds the physical directory string and all its
    subdirectories to the current |Coq| loadpath.
 
-   .. cmdv:: Add Rec LoadPath @string
-
-      Works as :n:`Add Rec LoadPath @string as @dirpath` but for the empty
-      logical directory path.
-
 
 .. cmd:: Remove LoadPath @string
 
@@ -784,12 +774,6 @@ the toplevel, and using them in source files is discouraged.
    loadpath (see the command `Declare ML Module`` in Section :ref:`compiled-files`).
 
 
-.. cmd:: Add Rec ML Path @string
-
-   This command adds the directory :n:`@string` and all its subdirectories to
-   the current OCaml loadpath (see the command :cmd:`Declare ML Module`).
-
-
 .. cmd:: Print ML Path @string
 
    This command displays the current OCaml loadpath. This