Merlin emacs mode does not support project-wide occurrences about merlin

[voodoos]

@xvw To tests your changes on a small scale you won't need the custom dune: you can build the index by hand and instruct Merlin to use it:

From the testsuite:

  $ cat >lib.ml <<'EOF'
  > let foo = "bar"
  > let () = print_string foo
  > EOF

  $ cat >main.ml <<'EOF'
  > let () = print_string Lib.foo
  > EOF

  $ ocamlc -bin-annot -bin-annot-occurrences -c lib.ml main.ml

  $ ocaml-index aggregate main.cmt lib.cmt
  $ ocaml-index dump project.ocaml-index
  2 uids:
  {uid: Stdlib.312; locs:
     "print_string": File "lib.ml", line 2, characters 9-21;
     "print_string": File "main.ml", line 1, characters 9-21
   uid: Lib.0; locs:
     "foo": File "lib.ml", line 1, characters 4-7;
     "foo": File "lib.ml", line 2, characters 22-25;
     "Lib.foo": File "main.ml", line 1, characters 22-29
   }, 0 approx shapes: {}, and shapes for CUS .

  $ $MERLIN single occurrences -scope project -identifier-at 1:28 \
  > -index-file project.ocaml-index \
  > -filename main.ml <main.ml
  {
    "class": "return",
    "value": [
      {
        "file": "$TESTCASE_ROOT/lib.ml",
        "start": {
          "line": 1,
          "col": 4
        },
        "end": {
          "line": 1,
          "col": 7
        }
      },
      {
        "file": "$TESTCASE_ROOT/lib.ml",
        "start": {
          "line": 2,
          "col": 22
        },
        "end": {
          "line": 2,
          "col": 25
        }
      },
      {
        "file": "$TESTCASE_ROOT/main.ml",
        "start": {
          "line": 1,
          "col": 26
        },
        "end": {
          "line": 1,
          "col": 29
        }
      }
    ],
    "notifications": []
  }

1. Build the cmt files with indexes in them with OCaml 5.2 and the -bin-annot -bin-annot-occurrences flags.
2. Gather the occurrences from all the cmts by running the indexer: ocaml-index aggregate -o the_index_file.index *.cmt
3. Instruct Merlin to use that index via the emacs merlin-flags command: -index-file ./the_index_file.index or by using a .merlin file with the directive INDEX ./the_index_file.index.

These are still moving parts, if by mistake the latest versions of these tools don't work together you can follow the instructions here for a working 4.14-base version of the feature: https://discuss.ocaml.org/t/ann-preview-play-with-project-wide-occurrences-for-ocaml/13814

The emacs mode should work regardless :-)