Add recursive package memory for quicker builds

[tylerjonesio]

This PR fixes #709. With large package graphs, running the skipstone build tool plugin could take a very long time and potentially cause OOM errors within Xcode. By adding recursive memory to the recursivePackageDependencies(for package: Package) and dependencies(for targetDependencies: [TargetDependency], in package: Package) functions, we can ensure the built tool doesn't needlessly traverse packages that have already been seen.

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• REQUIRED: I have signed the Contributor Agreement
• REQUIRED: I have tested my change locally with swift test
• OPTIONAL: I have tested my change on an iOS simulator or device
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• AI was used to generate or assist with generating this PR. Please specify below how you used AI to help you, and what steps you have taken to manually verify the changes.

Codex was used to help investigate performance issues and make a first pass at these improvements. I followed up with some code cleanup and tested against my own project. The built tool time went from ~300 seconds to 1 second.

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[marcprux]

This looks like a nice simple fix! How extensively have you exercised this on your own projects? Have you noticed any glitches (like stale builds)?

[tylerjonesio]

I've tested an earlier version of this off an on for a couple of weeks. I've tested this specific revision a little more heavily the past few days. I was a little nervous digging into this since this is such a key component, but so far I haven't noticed any stale builds or bad side effects. Just quicker builds!