Consider making `Evaluator#rootModule` public again

[lolgab]

This is used in plugins, like scala-steward plugin: https://github.com/scala-steward-org/mill-plugin/blob/77c1f6f444c2a1ffc7cb03c4937d39d403db66fa/plugin/src/org/scalasteward/mill/plugin/StewardPluginBase.scala#L29