scripts: ⚡ Bolt: Optimize getBuildPaths in dl_cleanup.py

[ManupaKDU]

💡 What: Replaced the O(NM) nested loop calling os.path.exists() with an O(M) pre-scan using os.scandir() and a dictionary cache.
🎯 Why: getBuildPaths was making excessive file system stat calls, checking for every package's existence across every subdirectory in build_dir/. For many packages, this I/O bottleneck significantly slows down the cleanup script.
📊 Impact: Reduces the time complexity from O(NM) I/O checks to O(M) directory scans and O(1) dictionary lookups per package, drastically speeding up execution on large build directories.
🔬 Measurement: Run scripts/dl_cleanup.py on a large populated build_dir and measure the total execution time before and after the change.

Signed-off-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

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PR created automatically by Jules for task 11918502149959167457 started by @manupawickramasinghe

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