JDK9: Support module-info.java

Ticket originates from discussion with Scala team how to handle the module-info.java of Jigsaw in Scala builds.

Scala will not get it’s own special syntax for these files, as @retronym correctly observed that the syntax is more of its “own thing”. Looks kind of like protobuf rather than any of scala/java, so seems we’ll use the same syntax.

• This means that mixed language projects will put the file into src/main/java, so sbt should be able to pick this up (not 100% sure yet what this implies, perhaps just a simple “compile it”)
• For just Scala projects though, it would be a pain to have to put and recreate package structure in parallel to the one in src/main/scala only to put the single module-info file there, it would also be a bit harder to find for people browsing the code. Therefore I’d propose to be able to notice that there’s a module-info.java file even if it lives under src/main/scala and treat it in that special way - (compile with javac I guess?)

This is lose thought for now, feel free to sanity check my assumptions and challenge ideas 😃

[retronym]

A long time ago, I put together some alpha-quality support for SBT+JPMS in https://github.com/retronym/sbt-jpms. Could be time to take another swing at this problem.

[SethTisue]

has anybody tried compileOrder := CompileOrder.JavaThenScala as a way of keeping scalac from blowing up trying to process the .java source?