Support decoding link summaries with absolute paths to external pages

[d-ronnqvist]

Bug/issue #, if applicable: rdar://149470919

Summary

Support decoding link summaries from external documentation sources that use absolute paths.

This for exa please allows interfacing with another documentation system that hosts its pages on a different domain.

Dependencies

None.

Testing

• Modify the bin/test-data-external-resolver script on line 95 to return an absolute web URL for the page.

• Set the DOCC_LINK_RESOLVER_EXECUTABLE environment to the path to the bin/test-data-external-resolver script

• Build some documentation with a link to <doc://com.test.bundle/something> in the content.

• Preview the documentation.

  • The link on the web page should have the full absolute web URL from the bin/test-data-external-resolver script.

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

[d-ronnqvist]

@swift-ci please test