remove duplicated logic

[heckj]

Originally found while debugging #1084, and recently updated the logic in swiftlang/swift-docc-symbolkit#101 to match this version, which was used in preference of the SymbolKit version.

Summary

This removes the logic from DocC, favoring the loading in SymbolKit, and consolidating the logic into a single place. This was originally opened at #1091, but the code had changed significantly since that update, so it was easier to create a new PR with the change after applying feedback from the earlier PR.

The equivalent logic that I'm consolidating towards is https://github.com/swiftlang/swift-docc-symbolkit/blob/main/Sources/SymbolKit/UnifiedSymbolGraph/GraphCollector.swift#L121-L178

Testing

The existing tests within DocC work this logic extensively.

Checklist

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

• Added tests - no additional tests needed
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary - no documentation changes are relevant, internal implementation details only

[heckj]

@swift-ci please test

[d-ronnqvist]

If the SymbolKit logic for this was recently updated, does it make sense to also run swift package update swift-docc-symbolkit to update the commit of SymbolKit that DocC depends on or do you prefer to update that separately?