Fix environment setup

[azzsal]

Currently, each corresponding pair of signatures are diffed in their own separate environment. This creates problems if one signature references an item outside of its scope. We need to have the environment setup only once before diffing signatures, and diff the signatures in this shared environment.

Minimal reproduction with .cmi files:

$ cat > ref.mli << EOF
> module M : sig
>  type t = int
> end
> module N : sig
>  val x : M.t
> end
> EOF

$ ocamlc ref.mli

$ cat > cur.mli << EOF
> module M : sig
>  type t = float
> end
> module N : sig
>  val x : M.t
> end
> EOF

$ ocamlc cur.mli

$ dune exec -- api-diff ref.cmi cur.cmi

No diff will be reported on val x : M.t

Minimal reproduction with wrapped libraries:

$ mkdir ref_lib && cd ref_lib
$ cat > dune-project << EOF
> (lang dune 2.9)
> EOF

$ cat > dune << EOF
> (library
>  (name library))
> EOF

$ cat > m.ml << EOF
> type t = int
> EOF

$ cat > n.ml << EOF
> type u = M.t
> EOF

$ dune build

$ cd .. && mkdir cur_lib && cd cur_lib
$ cat > dune-project << EOF
> (lang dune 2.9)
> EOF

$ cat > dune << EOF
> (library
>  (name library))
> EOF

$ cat > m.ml << EOF
> type t = float
> EOF

$ cat > n.ml << EOF
> type u = M.t
> EOF

$ dune build

$ cd .. && dune exec -- api-diff --main-module=library ../ref_lib/_build/default/.library.objs/byte ../cur_lib/_build/default/.library.objs/byte

Same as with .cmi files, there isn't a diff reported on type u = M.t, while there should be one.