Add some logic to the default value of EclipseKeys.createSrc

This could be used to determine if source-managed should be added to the project's source roots:

EclipseKeys.createSrc <<= ((sourceGenerators in Compile) { sourgeGens =>
if(sourgeGens == Nil)
EclipseCreateSrc.ValueSet(EclipseCreateSrc.Unmanaged, EclipseCreateSrc.Source)
else
EclipseCreateSrc.ValueSet(EclipseCreateSrc.Unmanaged, EclipseCreateSrc.Source, EclipseCreateSrc.Managed)
})

Also I wish there was a setting such as :

EclipseKeys.createSrcDirs : TaskKey[Seq[File]]

By default this setting would feed on the output of the existing EclipseKeys.createSrc unchanged.
It would give a possibility to add or remove the directories that were generated
by EclipseKeys.createSrc. I would use it to remove directories like src/main/java, src/test/java when
they are empty.

Start by tracing EclipseKeys.createSrc and sourceGenerators in Compile; the issue provides no file or test path to begin with. Done means deciding and implementing the conditional source-root behavior, plus determining whether a createSrcDirs setting can add or remove generated directories such as empty src/main/java and src/test/java.