A build context that matches a mavenproject to a given tasksegment, and the session to be used.
A note to the reader;
There are several issues/discussions regarding how "aggregator" plugins should be handled.
Read for instance http://docs.codehaus.org/display/MAVEN/Deterministic+Lifecycle+Planning
In their current implementation they are "bolted" onto the lifecycle by separating them
into TaskSegments. This class represents the execution context of one such task segment.
Wise voices have suggested that maybe aggregators shouldn't be bound to the ordinary
lifecycle at all, in which case we wouldn't be needing this class at all ( and
ProjectBuildList.getByTaskSegments). Or maybe they should be introduced in the calculation
of the execution plan instead, which seems much nicer.
Additionally this class contains a clone of the MavenSession, which is *only* needed
because it has as notion of a "current" project.