[jvm] awkward main class naming with module-level main

If you want to use `java -cp`, the name class name must be given explicitly, and that's quite awkward to use with a module-level `main()` field because of the naming:

```
java -cp _Main.Main_Fields_
```

Could we do something about that, e.g. have a special case for entry points and simply generate it as `Main`?

(for `-jar` the main name from the manifest is implied, but you can't use `-jar` and `-cp` at the same time)