Enable external link support by default

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This PR enables external link support by default in 3dab3b9 and then offsets the increased convert time from emitting the new data in a5f823e and 08f8202.

For one large mixed language project, the total convert time changed like this in these commits:

08f82024 ████████████████████████████████████████████████████████████████████████████████   15,072 sec
a5f823ed █████████████████████████████████████████████████████████████████████████████████▎   15,305 sec
3dab3b9d █████████████████████████████████████████████████████████████████████████████████████▍   16,075 sec
main     ████████████████████████████████████████████████████████████████████████████████   15,061 sec

Note

This PR only enables the DocC feature for the convert command by default. There's still more work in the merge command and in DocC package plugin before this feature is "complete".

Dependencies

None.

Testing

• Build any project.
  • DocC will write a "link-hierarchy.json" file and a "linkable-entities.json" file into the output archive by default.
    This makes the archive useable as a dependency (--dependency) when passed to another docc convert call.

Checklist

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

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

[d-ronnqvist]

Some of these conflicts are in files that will be removed by #1266

I'll rebase this sometime after that's been merged.

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

Note; this will remain in draft until it's been discussed in the Swift Forums.