Fix publishing for plugin cross versions

Earlier versions of 2.13 do not support -Wconf nor -release:8 (they
support "-release 8").

Author: jackkoenig

plugin/package.mill

@@ -13,16 +13,16 @@ object `package` extends RootModule {
   object cross extends Cross[Plugin](v.pluginScalaCrossVersions)
 }
 
-trait Plugin extends CrossSbtModule with ScalafmtModule with HasCommonOptions with release.ChiselPublishModule {
+trait Plugin extends CrossSbtModule with ScalafmtModule with release.ChiselPublishModule {
   override def artifactName = "chisel-plugin"
 
   def millSourcePath = super.millSourcePath / os.up
 
   // The plugin is compiled for every minor Scala version
   override def crossFullScalaVersion = true
 
-  // There is no code sharing in the plugin so no point in -Xsource:3
-  override def xsource3 = false
+  // Does not use common options because -Wconf wasn't supported in early 2.13, nor was "-release:"
+  override def scalacOptions = super.scalacOptions() ++ Seq("-release", "8")
 
   def scalaLibraryIvy = v.scalaLibrary(crossScalaVersion)
   def scalaReflectIvy = v.scalaReflect(crossScalaVersion)