=pro load sample builds with project dependencies

* load sample builds from their definitions and replace library dependencies with project ones
* remove redefined sample build definitions
* test osgi sample by running a maven command

This unblocks #16096 because after SBT 0.13.6 project definitions defined in sbt files take precedence over same projects defined in ```... extends Build {}``` definitions. This prevents us redefining sample build definitions in `AkkaBuild.scala`.

OSGi dining hakkers is tested by running maven command. I tried to write `.sbt` files for that sample but was hit by various OSGi related errors when running tests. Therefore I delegated for maven to do all the work.

This is one more step to the direction of #15863 and #15031.
Is there any downside to using ProjectRef everywhere for all projects? It has to be an external project file I assume... so it couldn't share Dependencies.scala for example etc? 

// background: I'm very tempted to try to implement something that would not build projects that in no way were touched by a given PR (neither as dependency, or source changes).