Combining scaladoc from multiple sources in mill

build.sc

// This module isn't really a ScalaModule, but we use it to generate
// consolidated documentation using the Scaladoc tool.
object docs extends ScalaModule {
  def scalaVersion = "3.0.0-M3"

  def docSource = T.source(millSourcePath)

  def moduleDeps = Seq(
    ...
  )

  // generate the static website
  def site = T {
    import mill.eval.Result

    for {
      child <- os.walk(docSource().path)
      if os.isFile(child)
    } {
      os.copy.over(child, T.dest / child.subRelativeTo(docSource().path), createFolders = true)
    }
    val files: Seq[os.Path] = T.traverse(moduleDeps)(_.allSourceFiles)().flatten.map(_.path)

    // format: off
    val options = Seq(
      "-classpath", compileClasspath().map(_.path).mkString(":"),
      "-siteroot", T.dest.toString,
      "-project-url", "<url>",
      "-project-logo", "logo.svg",
      "-project-version", "<version>",
      "-project", "<name>"
    ) ++ scalaDocPluginClasspath().map(pluginPathRef => s"-Xplugin:${pluginPathRef.path}")
    // format: on

    zincWorker.worker().docJar(
      scalaVersion(),
      scalaOrganization(),
      scalaDocClasspath().map(_.path),
      scalacPluginClasspath().map(_.path),
      files.map(_.toString) ++ options
    ) match{
      case true =>
        Result.Success(PathRef(T.dest / "_site"))
      case false =>
        Result.Failure("doc generation failed")
    }
  }

  // preview the site locally
  def serve() = T.command{
    os.proc("python3", "-m", "http.server", "--directory", site().path).call()
  }
}