Consider supporting groupIds based on embedded Maven metadata

[jesse-gallagher]

Some p2 repositories, like Eclipse's, contain Maven metadata inside the artifacts.xml and within the artifacts themselves, and it could make sense to use this when available.

The code would have to be careful to look up by artifactId alone first, and then verify that the requested group ID matches either the embedded one or (only if there is no embedded one present) the configured one for the repo.

[jesse-gallagher]

This will normally be pretty unneeded, since most artifacts with Maven metadata will also be available in Maven Central. However, it might be useful for IBM-style deliveries where they have an internal Maven structure that isn't represented externally, but they give out zipped update sites.