sbt

question.scala

// project/ServiceApiBuild.scala

object ServiceApiBuild {
 def idlProject(projectName : String) =
    Project(
      id = projectName,
      base = file(projectName)
    )
      .settings(name := projectName)
      .settings(
        artifact in (Compile, packageBin) := { 
        val art: Artifact = (artifact in (Compile, packageBin)).value
          art.copy(`classifier` = Some("idl"))
        }
      )
}

// build.sbt

lazy val `foobar-idl` =
  idlProject("foobar-idl")
    .settings(commonSettings: _*)