Add typesafe API to access the marker files

Description

It's useful to publish the markers without publishing everything

val publishMarkers = tasks.register("publishMarkers") {
  val from = tasks.named("librarianGenerateMarkerFiles").map { it.outputs.files }
  val into = rootProject.layout.buildDirectory.dir("localMaven")
  dependsOn("librarianGenerateMarkerFiles")
  doLast {
    val src = from.get().files.single()
    val dst = into.get().asFile
    publishFileByFile(FilesystemTransport(dst.absolutePath), src.walk().filter { it.isFile }.map { FileWithPath(it, it.relativeTo(src).path) }.toList())
  }
}

Start by locating the librarianGenerateMarkerFiles task and the APIs used in the example: publishFileByFile, FilesystemTransport, and FileWithPath. Trace how marker files are currently exposed, then determine the typesafe API needed to support the shown publishing flow; done means callers can access and publish the marker files without publishing everything else.