Any cover-image set in YAML header should be moved to output dir

[cderv]

It seems that when a cover-image is defined, it is not moved to the published folder.
It will only be if the image itself it used in the book body.

This should be improved IMO.

[cderv]

Using this unfortunately does not help

cover-image: cover.jpg
resource_files:
  - cover.jpg

bookdown may not be using rmarkdown::find_external_resources()