[Go] Generated lexer and parser file names are always in lower case with underscore

[KvanTTT]

The following command

Test.g4 -Dlanguage=Go

generates test_lexer.g4 and test_parser.g4 instead of TestLexer.g4 and TestParser.g4 like other runtimes.

[parrt]

I'm going to close this as it looks intentional.