Require built-in xref to parse cross-references

Otherwise we run into errors such as the one reported by Frank Ehmsen on
the mailing list:
<https://lists.sr.ht/~protesilaos/denote/%3C579a31b8-d1dd-5c02-aa2f-498526c5982e@eh-is.de%3E>.

denote-retrieve.el

@@ -30,6 +30,7 @@
 ;;; Code:
 
 (require 'denote)
+(require 'xref)
 
 (defconst denote-retrieve--title-front-matter-regexp
   "^\\(?:#\\+\\)?\\(?:title\\)\\s-*[:=]\\s-*[\"']?\\(?1:.*\\b\\)[\"']?"
@@ -97,8 +98,6 @@ Optional GROUP is a regexp construct for
                       (when (denote--only-note-p f) f))
                     files)))
 
-(declare-function xref--analyze "xref" (xrefs))
-
 (defun denote-retrieve--xrefs (identifier)
   "Return xrefs of IDENTIFIER in variable `denote-directory'.
 The xrefs are returned as an alist."