This introduces a convenience method on PackageBundle that looks up the

newest release matching a version range across all channels meeting the

specified criteria. The caller can specify the allowed channel type

(pre-release or release), the release types (alpha, beta, etc.), and

limit the result to the version range provided.
This is useful primarily for dependency processing.
As part of this, PackageChannel, and the serialization format, now

supports a channel type: "prerelease" or "release".

Unit tests pass.