easy support for sbt/maven-style directory layout

[cvogt]

plugins.sbtLayout is too annoying right now, this should be easy. Maybe at least make them built-in plugins

[cvogt]

It's actually become a built-in plugin now, meaning you can just mix in SbtLayout and you are good. But it's not been battle tested and we should do that.

[ShaneDelmore]

Works great in my testing.