Package pre-release search for latest version

I implemented this to assist with release branching prior to the next release of Octopus Deploy.  Basically, you can now specify any pre-release version and the Octo tool will use the latest version of that pre-release when creating a release.

This way, you can simply specify "--packageprerelease dev" when creating a release via Octo and it will use the latest packages under that pre-release (which we use for branching).

This probably won't be necessary once branching is a first-class citizen, but until then, we found it to be the only way.