`@require` not support relative paths

[ACTCD]

https://wiki.greasespot.net/Metadata_Block#@require

The URL specified may be relative to the URL the script is being installed from.

But we don't actually support this, and since the current architecture does not record installURL, we can't easily achieve this.

Related:

• Record installURL and normalize update logic #836
• Implement Subresource Integrity (SRI) security feature #584