Xref backend without tunneling

[xvw]

This PR make two things:

• Add ocaml-eglot-locate with a behaviour similar to merlin-locate (based on preference)
• Remove, when it is possible, usage of merlinCallCompatible for finding definition/declaration

[catern]

Can you try using eglot--lsp-xrefs-for-method for this case?

[xvw]

I try this implementation:

(cl-defmethod xref-backend-definitions ((_backend (eql ocaml-eglot-xref)) symbol)
  "Extension of `xref-backend-definitions' for SYMBOL."
  (if-let* ((pt (get-text-property 0 'eglot--lsp-workspaceSymbol symbol)))
      ;; SYMBOL is from `xref-backend-identifier-at-point',
      ;; since if it was read from the minibuffer its text
      ;; properties would have been stripped
      ;; (see `minibuffer-allow-text-properties').  Just pass
      ;; position and Merlin will figure out everything from that.
      (eglot--lsp-xrefs-for-method
       (if (eq ocaml-eglot-locate-preference 'mli)
           :textDocument/declaration
         :textDocument/definition))
    ;; The LSP doesn't support jumping to definition of an arbitrary identifier,
    ;; so we have to fallback on ocamllsp/locate.
    (eglot--lsp-xrefs-for-method
     :ocamllsp/locate
     :extra-params (list :kind (if (eq ocaml-eglot-locate-preference 'mli)
                                   "declaration"
                                 "definition")
                         :prefix (string-remove-suffix "." symbol))
     :capability :experimental/ocamllsp/handleLocate)))

But it leads to the following issue: [eglot] Unsupported or ignored LSP capability 'ocamllsp/handleLocate' and if I avoid capability it try to use locateProvider so it seems blocked.