(fix) Replace olp with heading list

[hwiorn]

Motivation for this change

Closes #2335

After org-roam-capture--fill-template which actually calls (kill-buffer) inside it, strangely kill-buffer reverts buffer position from (point-min) to original. So re-search-forward loop can't find headings correct and org-roam-capture-find-or-create-olp keep creating headings if the buffer is already visited and buffer position is under below the OLP.

So I change olp to (setq headings (mapcar #'org-roam-capture--fill-template olp)) then use it in the search loop.

After this patch, below test code doesn't make new "Tasks" heading again when it exists already.

(let ((org-roam-directory (expand-file-name org-roam-dailies-directory org-roam-directory))
      (org-roam-dailies-directory "./")
      (org-roam-dailies-capture-templates
       `(("t" "Tasks" entry "* %?"
          :if-new (file+head+olp "test.org" ;; "%<%Y-%m-%d>.org"
                                 "* Notes\n\n* Tasks\n\n* Journal"
                                 ("Tasks" "[2023-03-28 Tue]"))))))
  (save-window-excursion
    (org-roam-dailies--capture (current-time) t)))