properly splice package prefix for nested multi builds

Author: bishabosha

runner/src/mill/runner/MillBuildRootModule.scala

@@ -365,6 +365,7 @@ object MillBuildRootModule {
     val pkgLine = pkg.map(p => "package " + backtickWrap(p)).mkString("\n")
 
     if (isBuildOrModuleSc) {
+      val pkgPrefix = if (pkg.isEmpty) "`<empty>`" else pkg.map(backtickWrap).mkString(".")
       s"""$pkgLine
          |
          |import _root_.mill.runner.MillBuildRootModule
@@ -384,7 +385,7 @@ object MillBuildRootModule {
          |  )
          |}
          |import ${backtickWrap(miscInfoName)}.{millBuildRootModuleInfo, millBaseModuleInfo}
-         |package object ${backtickWrap(name)} extends _root_.`<empty>`.${backtickWrap(name + "_")}
+         |package object ${backtickWrap(name)} extends _root_.$pkgPrefix.${backtickWrap(name + "_")}
          |import ${backtickWrap(name)}._
          |class ${backtickWrap(name + "_")} extends $superClass {
          |