only use the last file component when deciding decoding strategy

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Bug/issue #, if applicable: rdar://112664398

Summary

Swift CI has started failing on Swift-DocC's tests, because its workspace folder includes an @ sign. This confuses the SymbolGraphLoader into thinking that there are no base-module symbol graphs, and thus refusing to use the concurrentlyAllFiles decoding strategy. This PR updates that logic to only inspect the last path component instead of the full file path, allowing these @ sign directories to successfully detect base-module and extension symbol graphs.

Dependencies

None

Testing

Steps:

1. Generate symbol graphs for multiple modules (or copy out two symbol graphs from the "Test Resources" directory) into a directory whose name contains @.
2. Run docc convert in a debugger and ensure that SymbolGraphLoader correctly uses the concurrentlyAllFiles decoding strategy to load symbol graph files in parallel.

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
• [ n/a ] Updated documentation if necessary

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@swift-ci Please test

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@swift-ci Please test

[ethan-kusters]

Looks great! This is making me question are file-path-name-based logic here in general but we would address that separately from this PR.

Thanks for jumping on this.

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@swift-ci Please test