fix root ostap.mlpack to avoid double-building

The meaning of `include` in ocamlbuild corresponds to an `open` in the
OCaml module system: the compilation units in the included directory
become available with their basename, without having to give their
directory as well. If you have a directory `foo/` with source file `foo/mod.ml`, using

     "foo": include

will make the module accessible as `Mod`, while with only

     "foo": traverse

it is known by ocamlbuild, but only with name `foo/Mod`.

Before this patch, the makefile passed flag `-I lib` to the last packing
step, which means that `Ostap` was an ambiguous name: it could be
produced from either the root `ostap.mlpack` or `lib/ostap.mlpack`. This
confused ocamlbuild digest checking (the caching information for both
packs is probably stored in the same place).

After this change, lib is not "include"-ed during the last packing
run, only traversed, and its modules are referred to explicit as, for
example, `lib/BNF3` in the root `ostap.mlpack`.

You can check that running `make` a second time doesn't perform any
compilation anymore.

Note: the reason why there are two mlpack of the same name is that the
`pa_ostap` syntax extension produces code referring to the `Ostap`
module directly. In a usual project one would simply rename
`lib/ostap.mlpack` into something like `lib/inner_ostap.mlpack`, to
avoid any confusion, but here it is crucial that both packs have the
same name to be able to the same syntax extension (provided for users)
to build the `Util` module (in presence of a simplified
`Ostap` module).