Fix using private heimdall API usage and rearranging graph.

[krisselden]

Tracking the broccoli graph and heimdall graph should be separate and just reference each other so that a tool can display the timings as they happened on a timeline and/or display the broccoli graph with its times associated instead of trying to rearrange the heimdall graph.

Do we need _reset() or can we just create a node to parent the build?

Update heimdalls traversal methods to not be recursive then use them instead of touching _children.