Keep external links to non-symbols in generated See Also sections

[d-ronnqvist]

Bug/issue #, if applicable: rdar://116540790

Summary

Fix a bug where only links to external symbols were included in the generated See Also sections.

Dependencies

None

Testing

This can be tested using bin/test-data-external-resolver as an external resolver:

1. Set DOCC_LINK_RESOLVER_EXECUTABLE to the test-data-external-resolver path
2. Set DOCC_HTML_DIR to swift-docc-render-artifact checkout path (/path/to/swift-docc-render-artifact/dist)
3. In a documentation catalog that curates some local page somewhere, also curate an external link <doc://com.test.bundle/something>

For example, with an Article.md file in Something.docc:

# Article

An article.

and a Root.md file:

# Root

@Metadata {
  @TechnologyRoot
}

Curate a local article and an external link

## Topics

- <doc:Article>
- <doc://com.test.bundle/something>

5. Build the documentation

swift run docc preview path/to/Something.docc

6. Preview the curated page. It should display the external link in a generated See Also section

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

[patshaughnessy]

Looks good to me!

[d-ronnqvist]

@swift-ci please test