Swap dependency to buildflow-plugin

Swap the dependency so that buildgraph-view is dependent of build-flow-plugin instead of build-flow-plugin being optionally dependent on buildgraph-view. This seems to be the better way to do it since that is how they actually are dependent on each other and buildgraph-view can access functions and classes in the build-flow-plugin that it might need to customize what is shown in the graph.
Why not optional? Build Flow's former dependency could trivially be made optional.