Add support for for dune wrapped libraries

[fantazio]

Fix #18

Reproducing the issue's example now works as expected:

steps to reproduce:

$ cat dune-project
(lang dune 2.0)
(package (name dead))
$ cat dune
(library (public_name dead) (modules lib_mod) (wrapped true))
(executable (public_name dead_bin) (modules dead_bin) (libraries dead))
$ cat lib_mod.ml
let dead () = print_endline "dead code"
let live () = print_endline "live code"
$ cat lib_mod.mli
val dead : unit -> unit
val live : unit -> unit
$ cat dead_bin.ml
let () = Dead.Lib_mod.live ()
(* let () = Lib_mod.live () *)

The dead_code_analyzer provides the following reports:

$ dune build
$ dead_code_analyzer _build/default/
Scanning files...
 [DONE]

.> UNUSED EXPORTED VALUES:
=========================
/home/fantasio/proj/wrapped/_build/default/lib_mod.mli:1: dead

Nothing else to report in this section
--------------------------------------------------------------------------------


.> UNUSED METHODS:
=================

Nothing else to report in this section
--------------------------------------------------------------------------------


.> UNUSED CONSTRUCTORS/RECORD FIELDS:
====================================

Nothing else to report in this section
--------------------------------------------------------------------------------

Key Changes

• Use the module name found in the .cm[it] rather than the unit of the filename. DeadType.fields rely on them.
• Use the "compilation unit" of the sourcepath when possible instead of the one of the .cm[it] file. This is compared with the unit of the source locations in DeadCommon.export.
• Improve the sourcepath lookup on .cmi by checking all the locations found in them and selecting the one with the source filename the closest to the cmi filename.

With this change, the tool gives results on Frama-C (#23)