How to resolve "Referenced error: Highcharts not defined" when using Easychart?
Easychart 7.x-3.7 recommends version 4.2.7 of the Highcharts library. This is also what you can see in this specific commit.

And the Easychart project page states "... v4.x of the Highcharts library".

So it appears that using Highcharts 5.0.3 is not yet supported, which probably also explains the folder problems you described in your own answer. I don't think that you're missing anything else (or some other "latest" module) that you're missing (as long as you stick to the recommended version of Highcharts you should be fine).

PS: Be aware that there is also the feature request about "make use of new Highcharts 5.0.0 methods" (for the Easychart plugin).