Use static concurrent cache for dedupe.

[anuraaga]

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I first tried changing the Map to concurrent but it didn't fix the issue, making it static did though. If making it static has potential issues, an alternative might be something like attaching the provisioner to Project in newDedupingProvisioner to reference to dedupe the deduper itself, which was probably the root problem. I have left the map as concurrent though since I don't think Gradle's threading model is clear enough to safely use a non-concurrent one here.

I have run my build with a local snapshot and could confirm it passes with this change.

Fixes #1015

[nedtwigg]

This is a good fix, but I think it masks the root cause. Superceded by #1020. If you want the performance increase that this centralized approach provides, happy to take a PR for #984, but I'm skeptical that it would actually be faster.