Include images with Export disabled in project packages

Currently, packages saved using the "Package Project..." button exclude images used by image attachments when the `Export` option is disabled for those attachments.

However, there may be images that are not needed in the final texture atlas but are still useful to keep in the project for reference. When sharing a project with another person, it may be important for those images to be included in the package as well.

It would be more convenient and reasonable for project sharing if images were included in the package even when `Export` is disabled for their attachments.

**Related forum thread:**
https://esotericsoftware.com/forum/d/30560-feature-request-include-images-in-packages-even-when-export-is-disabled

Start by locating the implementation behind the "Package Project..." button and tracing how attachment Export settings affect package contents. Verify that packages include images used by attachments with Export disabled, while the existing export behavior remains unchanged.

Written by the indexing model from the issue text.