Support better directory structure

[rahulmutt]

@carymrobbins suggested that the directory structure should look something like this:

example/
├── example.cabal
├── Setup.hs
└── src/
    ├── main/
    │   ├── eta/
    │   │   └── Example/
    │   │       └── Transform.hs
    │   └── java/
    │       └── Main.java
    └── test/
        └── eta/
            └── Example/
                └── TransformTest.hs

The layout is a lot better than the existing layout, which places the .cabal file inside of src/main/eta.

[carymrobbins]

I always forget this, but the Main.java probably needs to go in the src/main/eta directory. Build tools usually try to compile all of the src/main/java sources with just javac and before any other sources, so if Main.java references anything from src/main/eta it'll fail to compile. Maybe sbt is different, but either way this is a potential gotcha.

[rahulmutt]

So in these inter-project setups, there's always the question of where the dependencies flow. SBT has a mechanism which allows you to select whether you want to compile Scala or Java dependencies first. We probably need to add a new option that chooses the order among, Java, Scala, Eta (in case anybody wants such complicated setups).

[carymrobbins]

In this particular case, it would make sense to disable javac building altogether and add the following to the cabal file -

java-sources: src/main/java/Main.java

or

java-sources: src/main/eta/Main.java

Whichever was preferred (the latter of which would not require disabling or changing when the javac build happens).