text: Title: Default to read-only field on import if a single Resource or file format is defined
  Body: **Describe the bug**
  
  Export allows for single elements to be pre-selected (added in #1671):
  
  ![image](https://github.com/django-import-export/django-import-export/assets/6249838/f1d4005a-27e6-43dc-b623-18d6c4bb0223)
  
  However this behaviour is not supported on the import page:
  
  ![image](https://github.com/django-import-export/django-import-export/assets/6249838/87b4a7cd-3a82-4dc5-bbba-082d5237c8c3)
  
  **To Reproduce**
  
  Import / Export specified model
  
  **Versions (please complete the following information):**
   - Django Import Export: 4.0
   - Python 3.11
   - Django 4.2
  
  <s>Perhaps if there is only ever one resource and file format, this page is redundant and can be skipped?</s>
  This doesn't apply because user still has to choose file to import.
  
  