Binding codegen should not tolerate duplicate files

Duplicate classes are indicative of a mapping conflict and we should not merely warn on them, but rather fail the build unless instructed otherwise.

Refactor org.opendaylight.mdsal.binding.java.api.generator.GeneratorJavaFile and org.opendaylight.mdsal.binding.maven.api.gen.plugin.CodeGeneratorImpl to perform two-stage file/codegen mapping so that they lazily map files/content.

GeneratorJavaFile.generateToFile() needs to be updated to return Map<File, CharSource>, which will contain unique mapping. It should take a flag indicating whether conflicts should cause an abort or should be ignored. CodeGeneratorImpl needs to then perform the actual generation output and route maven configuration to that flag.