Return a File descriptor for vocab file. Look in library or
in -o output path. antlr -o foo T.g4 U.g4 where U needs T.tokens
won't work unless we look in foo too. If we do not find the
file in the lib directory then must assume that the .tokens file
is going to be generated as part of this build and we have defined
.tokens files so that they ALWAYS are generated in the base output
directory, which means the current directory for the command line tool if there
was no output directory specified.