Swap dependency to buildgraph-view

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.

Closely related to: https://github.com/sjoroos/buildgraph-view/commit/53de75a6f345a639941c274aff819f0aa889e47c