[entropy.client.interfaces.package] split Package class, create new API

The Package has been split into multiple PackageAction based classes.
The new interface uses a factory (PackageActionFactory) to build
the PackageAction objects on behalf of the caller.

The new factory-based interface is clean, modular and faster than
the old Package spaghetti class.

Backward compatibility has been preserved, as long as caller doesn't
peek into internal structures and metadata. The old interface has
been deprecated and a new Entropy Client method called PackageActionFactory()
has been added.

Please migrate your code to the new API, the old API will be dropped in 1
solar year.