Optimize exportparts tree with cache

[aomarks]

We should be able to cache the exportsparts tree, maybe directly on each node, so that we don't need to walk the same exporting part ancestor more than once.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale[bot]]

This issue has been automatically closed after being marked stale. If you're still facing this problem with the above solution, please comment and we'll reopen!