Subtree anchors get set to the page/file ID if using org-id as anchors

[kaushalmodi]

Test cases are in #542 (comment) .

The issue can be reproduced after evaluating

(with-eval-after-load 'ox-hugo
  (setq org-hugo-anchor-functions '(org-hugo-get-custom-id
                                    org-hugo-get-id
                                    org-hugo-get-heading-slug
                                    org-hugo-get-md5)))

and exporting this file:

:PROPERTIES:
:ID:       31c61d47-0afc-4d5c-9b60-6c154a1c518d
:END:
#+title: PL
#+hugo_base_dir: .
#+options: author:nil

* Features
** TSL
:PROPERTIES:
:ID:       48e6dfd4-93d9-4811-855e-c739470e83d1
:END:

Current markdown output

+++
title = "PL"
draft = false
+++

## Features {#31c61d47-0afc-4d5c-9b60-6c154a1c518d}


### TSL {#31c61d47-0afc-4d5c-9b60-6c154a1c518d}

Expected markdown output

+++
title = "PL"
draft = false
+++

## Features {#features}


### TSL {#48e6dfd4-93d9-4811-855e-c739470e83d1}