add logic to properly handle cross-import overlay graphs

[heckj]

Follow up from https://github.com/swiftlang/swift-docc/pull/1091/files#r2298429213 to address missing logic to accommodate a symbol graph from a cross-import overlay. The additional logic isn't yet matched with tests that verify the results in a loading scenario

[heckj]

@swift-ci please test

[QuietMisdreavus]

🤘

[heckj]

@swift-ci please test

[shahmishal]

@heckj we are seeing a test failure, however not sure if its relates to this change.

16:43:16  Test Case '-[SwiftDocCTests.RenderMetadataTests testRendersBystanderExtensionsFromSymbolGraph]' started.
16:43:16  /Users/ec2-user/jenkins/workspace/oss-swift-pr-test-macos-arm64/swift-docc/Tests/SwiftDocCTests/Rendering/RenderMetadataTests.swift:131: error: -[SwiftDocCTests.RenderMetadataTests testRendersBystanderExtensionsFromSymbolGraph] : XCTUnwrap failed: expected non-nil value of type "DocumentationNode"
16:43:16  Test Case '-[SwiftDocCTests.RenderMetadataTests testRendersBystanderExtensionsFromSymbolGraph]' failed (0.844 seconds).
16:43:16  Test Suite 'RenderMetadataTests' failed at 2025-08-28 23:43:05.027

https://ci.swift.org/view/Apple%20Silicon%20(main)/job/oss-swift-pr-test-macos-arm64/128/console

[heckj]

I've reproduced the error locally, working to debug and understand how this is impacting the loading in the DocC tests, and making sure that SymbolKit's loader is correctly aligned here.

[heckj]

fixes for the logic that resolve the DocC tests: #102