Improve caching of artifacts using new Transport#downloadArtifact

[laeubi]

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Currently Tycho maintain different caches, e.g. some items are cached in the local maven repository, some are cached in the transport caches and there is even a P2 remote cache maintained. Apart from that, P2/PDE/Ooomph also maintain so called bundle pools.

With the new Transport#downloadArtifact method we now have an opportunity to merge all these caches in a reliable way just using the artifact that is already there saving both downloads times and disk space.