dependency mediation and conflict management

as previously planned, we need to introduce the concepts of dependency mediation and conflict management to complement the transitive dependency mechanism.

Currently, a "nearest wins" approach is being taken which gives full control, but can result in having to re-state runtime dependencies to force a specified newer version in some instances.

An example in the m2 build is that when maven-artifact requires a newer version of wagon, it is not applied to core because plexus-artifact-container depends on an older version.

We should also only resolve a dependency once - for example, currently we build up the tree from all the versions of maven-artifact wanted, when in fact, we should decide what version of maven-artifact will be used, and use that for everything using maven-artifact before traversing to the next level. This would reduce the number of old dependencies appearing.

This will continue to make mboot less useful without replicating the feature. I suggest that we only bootstrap maven-artifact (or move it to wagon and don't bootstrap it at all), and then use it and wagon to execute mboot itself.