Remove custom script for lsp-elixir server. (#2764)

- Rename `lsp-clients-elixir-server-executable` to
 `lsp-elixit-server-command`.
 - Mark `lsp-clients-elixir-server-executable` as obsolete.
 - Define `elixir-ls` as lsp dependency.
 - Update how new connection is done for elixir-ls.

clients/lsp-elixir.el

@@ -92,15 +92,15 @@ If value is `\"\"` then defaults to the workspace rootUri."
   :group 'lsp-mode
   :link '(url-link "https://github.com/elixir-lsp/elixir-ls"))
 
-(defcustom lsp-clients-elixir-server-executable
-  (if (equal system-type 'windows-nt)
-      "language_server.bat"
-    "language_server.sh")
-  "The elixir-language-server executable to use.
-Leave as just the executable name to use the default behavior of
-finding the executable with `exec-path'."
+(define-obsolete-variable-alias 'lsp-clients-elixir-server-executable 'lsp-elixir-server-command "2021-04-05")
+
+(defcustom lsp-elixir-server-command '("elixir-ls")
+  "Command to start elixir-ls.
+
+Leave as default to let `executable-find' search for it."
   :group 'lsp-elixir
-  :type 'file)
+  :type '(repeat string)
+  :package-version '(lsp-mode . "7.1"))
 
 (defcustom lsp-elixir-enable-test-lenses t
   "Suggest Tests."
@@ -129,6 +129,9 @@ finding the executable with `exec-path'."
              " --no-color"))
     file-path))
 
+(lsp-dependency 'elixir-ls
+                '(:system "elixir-ls"))
+
 (lsp-register-custom-settings
  '(("elixirLS.dialyzerEnabled" lsp-elixir-dialyzer-enabled t)
    ("elixirLS.dialyzerWarnOpts" lsp-elixir-dialyzer-warn-opts)
@@ -142,7 +145,12 @@ finding the executable with `exec-path'."
    ("elixirLS.enableTestLenses" lsp-elixir-enable-test-lenses t)))
 
 (lsp-register-client
- (make-lsp-client :new-connection (lsp-stdio-connection (lambda () `(,lsp-clients-elixir-server-executable)))
+ (make-lsp-client :new-connection (lsp-stdio-connection
+                                   (lambda ()
+                                     `(,(or (executable-find
+                                            (cl-first lsp-elixir-server-command))
+                                           (lsp-package-path 'elixir-ls))
+                                       ,@(cl-rest lsp-elixir-server-command))))
                   :major-modes '(elixir-mode)
                   :priority -1
                   :server-id 'elixir-ls