Move part of the fix from project to xref

* lisp/progmodes/project.el (project--find-regexp-in-files):
Don't unquote file names here.

* lisp/progmodes/xref.el (xref-matches-in-files): Do it here.
And only if the first element in the list is quoted (bug#47799).

lisp/progmodes/project.el

@@ -787,11 +787,7 @@ pattern to search for."
 (defun project--find-regexp-in-files (regexp files)
   (unless files
     (user-error "Empty file list"))
-  (let ((xrefs (xref-matches-in-files
-                regexp
-                ;; FIXME: `xref-matches-in-files' should work with
-                ;; quoted filenames.
-                (mapcar #'file-name-unquote files))))
+  (let ((xrefs (xref-matches-in-files regexp files)))
     (unless xrefs
       (user-error "No matches for: %s" regexp))
     xrefs))

lisp/progmodes/xref.el

@@ -1534,6 +1534,8 @@ FILES must be a list of absolute file names."
                        #'tramp-file-local-name
                        #'file-local-name)
                    files)))
+    (when (file-name-quoted-p (car files))
+      (setq files (mapcar #'file-name-unquote files)))
     (with-current-buffer output
       (erase-buffer)
       (with-temp-buffer