Ensure that synthesized landing pages for combined archives display page images and links in abstracts from referenced types

[d-ronnqvist]

Bug/issue #, if applicable: rdar://147392201

Summary

This includes the dependency references of the merged archives in the synthesized landing page so that modules with page images display those images on the landing page:

Dependencies

None

Testing

For a project with multiple targets, for example https://github.com/d-ronnqvist/combined-documentation-example

• Add images and @PageImage directives to one or more of the documented products.

• Add a link in the abstract of one or more of the documented products.

• Build a local docc executable from this branch and specify its path for the DOCC_EXEC environmental variable.

• Build a combined documentation archive using swift package generate-documentation --enable-experimental-combined-documentation

• Start a preview server and preview the combined archives's landing page.

  • Both the page images and the abstract links should display on the landing page. Note that the entire card is a single element, so the abstract link isn't clickable.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• [ ] Updated documentation if necessary