Currently RequiredPluginsClasspathContainer itself implements
IClasspathContainer but we actually only need that instance to compute
the entries.

As the RequiredPluginsClasspathContainer retains some objects in its
instance and to make handling more uniform (e.g. regarding equals and
hashcode of container instances) this now removes the actual
implementation of the interface and simply wraps the resulting entries
in a SerializableClasspathContainer that only holds a reference to the
entries produced itself.