Avoid flattening of subpackages during convertion in featuremodel

During testing of various valid scenarios it became apparent that certain dependency scenarios for nested subpackages might start failing. If packages start to depend on each other or even deeper nested packages depend on each other we currently do not preserve the right order when flattening).

Instead the subpackages should be extracted (like bundles) and the implicit dependency to the (converted) parent package be made explicit (add dependency if not already existing.

The solution would mean:

• subpackages are extracted from the package and no longer handled as subpackages
• the subpackages get an additional explicit dependency on the parent package (that was previously implicitly present)
• the feature model contains all the converted packages and subpackages instead of a flattened one
• converted package and converted subpackages end up in the defined cache folder