Use unique out path for private named tasks - fixes #2107 (#2108)

This change ensures, we always use the `super`-`out` path when we try to
store metadata and scratch files for private named tasks. As private
tasks can't be seen from other traits/classes in the same module
hierarchy, our previous logic to detect potential collisions of the
metadata location fails. Hence we now keep the `isPrivate` property in
the named target.

I also added a test case that reproduce the initial issue but now
succeeds.

* Fixes: #2107

This is a binary compatibility breaking change.

Pull request: #2108

ci/mill-bootstrap.patch

@@ -0,0 +1,119 @@
+diff --git a/build.sc b/build.sc
+index be462bb7c..50eb14412 100644
+--- a/build.sc
++++ b/build.sc
+@@ -2,22 +2,10 @@
+ import $file.ci.shared
+ import $file.ci.upload
+ import $ivy.`org.scalaj::scalaj-http:2.4.2`
+-import $ivy.`de.tototec::de.tobiasroeser.mill.vcs.version_mill0.10:0.3.0`
+-import $ivy.`com.github.lolgab::mill-mima_mill0.10:0.0.13`
+ import $ivy.`net.sourceforge.htmlcleaner:htmlcleaner:2.25`
+
+ // imports
+-import com.github.lolgab.mill.mima
+-import com.github.lolgab.mill.mima.{
+-  CheckDirection,
+-  DirectMissingMethodProblem,
+-  IncompatibleMethTypeProblem,
+-  IncompatibleSignatureProblem,
+-  ProblemFilter,
+-  ReversedMissingMethodProblem
+-}
+ import coursier.maven.MavenRepository
+-import de.tobiasroeser.mill.vcs.version.VcsVersion
+ import mill._
+ import mill.define.{Command, Source, Sources, Target, Task}
+ import mill.eval.Evaluator
+@@ -180,12 +168,8 @@ object Deps {
+   val requests = ivy"com.lihaoyi::requests:0.8.0"
+ }
+
+-def millVersion: T[String] = T { VcsVersion.vcsState().format() }
+-def millLastTag: T[String] = T {
+-  VcsVersion.vcsState().lastTag.getOrElse(
+-    sys.error("No (last) git tag found. Your git history seems incomplete!")
+-  )
+-}
++def millVersion: T[String] = T { "0.0.0.test" }
++def millLastTag: T[String] = T { "0.0.0.test" }
+ def millBinPlatform: T[String] = T {
+   val tag = millLastTag()
+   if (tag.contains("-M")) tag
+@@ -244,20 +228,7 @@ trait MillCoursierModule extends CoursierModule {
+   )
+ }
+
+-trait MillMimaConfig extends mima.Mima {
+-  override def mimaPreviousVersions: T[Seq[String]] = Settings.mimaBaseVersions
+-  override def mimaPreviousArtifacts =
+-    if (Settings.mimaBaseVersions.isEmpty) T { Agg[Dep]() }
+-    else super.mimaPreviousArtifacts
+-  override def mimaExcludeAnnotations: T[Seq[String]] = Seq(
+-    "mill.api.internal",
+-    "mill.api.experimental"
+-  )
+-  override def mimaCheckDirection: Target[CheckDirection] = T { CheckDirection.Backward }
+-  override def mimaBinaryIssueFilters: Target[Seq[ProblemFilter]] = T {
+-    issueFilterByModule.getOrElse(this, Seq())
+-  }
+-  lazy val issueFilterByModule: Map[MillMimaConfig, Seq[ProblemFilter]] = Map()
++trait MillMimaConfig extends Module {
+ }
+
+ /** A Module compiled with applied Mill-specific compiler plugins: mill-moduledefs. */
+@@ -1549,53 +1520,7 @@ def launcher = T {
+ }
+
+ def uploadToGithub(authKey: String) = T.command {
+-  val vcsState = VcsVersion.vcsState()
+-  val label = vcsState.format()
+-  if (label != millVersion()) sys.error("Modified mill version detected, aborting upload")
+-  val releaseTag = vcsState.lastTag.getOrElse(sys.error(
+-    "Incomplete git history. No tag found.\nIf on CI, make sure your git checkout job includes enough history."
+-  ))
+-
+-  if (releaseTag == label) {
+-    // TODO: check if the tag already exists (e.g. because we created it manually) and do not fail
+-    scalaj.http.Http(
+-      s"https://api.github.com/repos/${Settings.githubOrg}/${Settings.githubRepo}/releases"
+-    )
+-      .postData(
+-        ujson.write(
+-          ujson.Obj(
+-            "tag_name" -> releaseTag,
+-            "name" -> releaseTag
+-          )
+-        )
+-      )
+-      .header("Authorization", "token " + authKey)
+-      .asString
+-  }
+-
+-  val exampleZips = Seq("example-1", "example-2", "example-3")
+-    .map { example =>
+-      os.copy(T.workspace / "example" / example, T.dest / example)
+-      os.copy(launcher().path, T.dest / example / "mill")
+-      os.proc("zip", "-r", T.dest / s"$example.zip", example).call(cwd = T.dest)
+-      (T.dest / s"$example.zip", label + "-" + example + ".zip")
+-    }
+-
+-  val zips = exampleZips ++ Seq(
+-    (assembly().path, label + "-assembly"),
+-    (launcher().path, label)
+-  )
+-
+-  for ((zip, name) <- zips) {
+-    upload.apply(
+-      zip,
+-      releaseTag,
+-      name,
+-      authKey,
+-      Settings.githubOrg,
+-      Settings.githubRepo
+-    )
+-  }
++  // never upload a bootstrapped version
+ }
+
+ def validate(ev: Evaluator): Command[Unit] = T.command {