Provide a unique output dir per set of source files

[albertocavalcante]

marcohu#23 (comment)

This is an attempt to make multiple rules_antlr invocations work without sandboxing (eg on Windows, where there's no sandbox). Test with --spawn_strategy=local.

Before, the AntlrRules.java code would fail, as it'd find generated grammar files without sandboxing it didn't know about.
Fix this by providing a unique directory per grammar that's being compiled.
The code that throws the exception is master/src/main/java/org/antlr/bazel/AntlrRules.java#L619.