Improve the dependency system

[Pierre Labastie]

Currently, the dependencies are only installed if either:

• (i) they are not already installed
• (ii) their version is higher than the installed version

If a dependency is not installed, its dependencies are not considered.

But it may happen that a dependency (depB) of a dependency (depA) is in case (i) or (ii) (most likely), but not depA. In this case, depB is not considered, although it might happen that it is needed for the requested package.

So the dependency system should be changed to always compute the full dependency chain, and another function should be created to remove packages not in case (i) or (ii) from the list of generated dependencies.