[toplevel] Refactor load path handling.

We refactor top-level load path handling. This is in preparation to
make load paths become local to a particular document.

To this effect, we introduce a new data type `coq_path` that includes
the full specification of a load path:

```
type add_ml = AddNoML | AddTopML | AddRecML

type vo_path_spec = {
  unix_path : string;           (* Filesystem path contaning vo/ml files *)
  coq_path  : Names.DirPath.t;  (* Coq prefix for the path *)
  implicit  : bool;             (* [implicit = true] avoids having to qualify with [coq_path] *)
  has_ml    : add_ml;           (* If [has_ml] is true, the directory will also be search for plugins *)
}

type coq_path_spec =
  | VoPath of vo_path_spec
  | MlPath of string

type coq_path = {
  path_spec: coq_path_spec;
  recursive: bool;
}
```

Then, initialization of load paths is split into building a list of
load paths and actually making them effective. A future commit will
make thus the list of load paths a parameter for document creation.

This API is necessarily internal [for now] thus I don't think a
changes entry is needed.

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