Compilation order not honored with features required later on in the depency graph

The following discussion from !324 (closed) should be addressed:

• @kartmann started a discussion: (+4 comments)

  Are there any use-cases where PKD needs to be built without those packages?

  For general use, PKD does not need to be built at all, just cmaked. That's why I would like to have the dependencies as optional, but I see the problems with the build.

  @dreher If we add RobotAPI and VisionX as feature dependencies to PKD, would that mean that the dependency is respected if the modules are present in the workspace? Need to test that ...