Revert "Install: stop pooping into ocaml's library directory."

This reverts commit b2d98ea.

We can't do that before opam switches are configured
correctly see:

ocaml/opam-repository#25819

B0.ml

@@ -62,6 +62,11 @@ let default =
         "ocamlbuild", {|build|};
         "topkg", {|build & >= "1.0.3"|};
         "uucp", {|dev|}]
+    |> B0_meta.add B0_opam.install {|
+      # Following is only to deal with
+      # https://caml.inria.fr/mantis/view.php?id=7808
+      [["install" "-d" "%{lib}%/ocaml/"]
+       ["install" "src/down.top" "%{lib}%/ocaml/"]]|}
   in
   B0_pack.make "default" ~doc:"down package" ~meta ~locked:true @@
   [down_lib]

CHANGES.md

@@ -1,17 +1,12 @@
-  
+
 - Restore (and improve) support for `ocamlnat`.  It's now possible to
   use `#use "down.top"` regardless. So if you don't have fancy stuff
   in your `.config/ocaml/init.ml` it will work equally with `ocaml`
   and `ocamlnat`. 
 
   Incidentally we no longer need to detect the presence of `ocamlnat`
   which solves #33 (Thanks to Jacques-Henri Jourdan for the report).
-
-- opam packaging: no longer install `down.top` in `ocaml`'s library
-  directoy. `OCAML_TOPLEVEL_PATH` is supported since 4.08 and `opam
-  env` sets it correctly so the version we install the `toplevel`
-  library directory should be enough.
-
+
 - Fix incorrect permission when creating directories (#36,
   v0.3.0 regression). Thanks to Juneyoung Lee for the report
   and Nicolás Ojeda Bär for the analysis.

opam

@@ -37,4 +37,8 @@ depends: [
 build: [
   "ocaml" "pkg/pkg.ml" "build" "--dev-pkg" "%{dev}%" "--lib-dir" "%{lib}%"
 ]
+install: [
+  ["install" "-d" "%{lib}%/ocaml/"]
+  ["install" "src/down.top" "%{lib}%/ocaml/"]
+]
 dev-repo: "git+https://erratique.ch/repos/down.git"