Created by: jbarr21
Since the Layout Preview uses compiler output of modules, not the module dependencies, this diff adds the ability to generate extra modules. This is useful for generating an extra module for each Android Library module so that the generated R.java class files can be added to the compiler output path. Doing this allows IntelliJ's Android Layout Preview to resolve the R file classes and properly load views in the Preview tab.
This approach was necessary since Buck stores compiled module class files in a different directory than the compiled dummy R.java class files and IntelliJ only support one compiler output path per module. This functionality is put behind a .buckconfig flag.