[generator] change swift-specific namespace from 'swift' to 'java.swift'
[generator] Add a switch to gate usage of plain java namespace

Instead of always looking for a swift-specific namespace in a .thrift file, and always falling back to the java namespace in case there is no swift-specific namespace, after this change we'll only look for one or the other (by default we'll use the swift-specific namespace, but we'll use java if '-use_java_namepsace' is passed on the command-line)
[generator] Allow namespace selection from maven plugin