- consider version of dependencies and return Sets of BundleDescriptions
instead of only bundle id/symbolic-name strings. This way multiple
versions of the same bundle are considered.

+ Implement the dependency computation based on an exhaustive
Breadth-first search to compute the self contained closure of all
required bundle-wires of an initially given set of plug-in models.

 - simpler algorithm (more abstract, thus less cases to consider)
 - improved runtime behavior, to be linear to the number of
   requirements and wires in the resulting closure. The later is likely
   to be dominating. Assuming all called methods have constant time.

+ (internal) API is changed to be collection based
+ returned Set is NOT sorted anymore

Change-Id: I7e78ab9efdbd79d757c852b6ccb42661df4a37ed
Signed-off-by: Hannes Wellmann <wellmann.hannes1@gmx.net>
Reviewed-on: https://git.eclipse.org/r/c/pde/eclipse.pde.ui/+/183468
Tested-by: PDE Bot <pde-bot@eclipse.org>
Tested-by: Julian Honnen <julian.honnen@vector.com>
Reviewed-by: Julian Honnen <julian.honnen@vector.com>