Replace utop-complete with a dispatch function

This function either sends the request for completion to utop and lets
the company backend take care of the rest, or calls
`completion-at-point`.

src/top/utop.el

@@ -721,9 +721,8 @@ If ADD-TO-HISTORY is t then the input will be added to history."
       (utop-send-string (concat "data:" line "\n")))
     (utop-send-string "end:\n")))
 
-(defun utop-complete ()
-  "Complete current input."
-  (interactive)
+(defun utop-complete-start ()
+  "Conditionally begins to request completion candidates from utop."
   ;; Complete only if the cursor is after the prompt
   (when (and (eq utop-state 'edit) (>= (point) utop-prompt-max))
     ;; Use this buffer
@@ -735,7 +734,7 @@ If ADD-TO-HISTORY is t then the input will be added to history."
 (defun utop-completion-at-point ()
   "Complete thing at point."
   (setq utop-capf-completion-candidates nil)
-  (utop-complete)
+  (utop-complete-start)
 
   (let ((elapsed-time 0))
     (while (and (eq utop-state 'comp)
@@ -749,6 +748,13 @@ If ADD-TO-HISTORY is t then the input will be added to history."
      (point)
      utop-capf-completion-candidates)))
 
+(defun utop-complete ()
+  "Complete current input."
+  (interactive)
+  (if (utop--supports-company)
+      (utop-complete-start)
+    (completion-at-point)))
+
 ;; +-----------------------------------------------------------------+
 ;; | Eval                                                            |
 ;; +-----------------------------------------------------------------+