Breaks file by removing clip even when child is referenced by a clone (`<use>`)

[nathanal]

Downstream report: https://gitlab.com/inkscape/extensions/-/issues/468

Steps to Replicate

• Run min_example_lost_clone through scour
  • scour -i min_example_lost_clone

What happened?

• The object the <use> element links to is deleted (because it is in a unused clone)

What should have happened?

• Object is not deleted.
• Ideally the clone is removed but relevant children preserved; however, keeping the clip is better than current behavior.

Input

min_example_lost_clone

<?xml version="1.0" encoding="UTF-8"?>
<svg version="1.1" viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
 <defs>
  <clipPath id="unusedclip">
  <rect id="the_path" width="100" height="100"/>
  </clipPath>
 </defs>
 <use fill="red" width="100%" height="100%" xlink:href="#the_path"/>
</svg>

Output

<?xml version="1.0" encoding="UTF-8"?>
<svg version="1.1" viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
 <use width="100%" height="100%" fill="red" xlink:href="#the_path"/>
</svg>

Version info

• Testing by cloning this repository and pip3 install . (I edited the scour version to make sure I could see it was properly installed) (0609c596766ec98e4e2092b49bd03b802702ba1a)
• Also happens with python3-scour from repos (0.37) and 0.38.2 from pip3
• Python 3.8.10, Linux Mint 20