[generator] Add a switch to gate fallback to 'namespace java ...'

Instead of always looking for a swift-specific namespace in a .thrift file, and always falling back to the java namespace if it doesn't exist, after this change we'll only use the swift-specific namespace unless a switch is thrown to enable fallback.
[generator] use 'java.swift' namespace instead of 'swift'

This is for clarity when reading a .thrift file that swift is not a new language, but a new implementation of java thrift.