Add proper package handling for antlr4 targets

Based on conversation with the antlr guys, it looks like having a package declared in a @header{} is heavily discouraged. Instead, it should be a flag to the Tool class when it is invoked. This allows the user to set it in the java_antlr_library target, or to let it be auto-set based on the path. It assumes that all of the paths are homogenous.