Cut dependency from website.fastPages on slow build tasks (#6045)

It now no longer depends on:

- `example.thirdparty[android-compose-samples].downloadedRepo` 
- `build.website.apidocs.unidocLocal`

This should speed up local iteration on the docsite. Anyone who wants
the fuller website can use `website.localPages` or `website.githubPages`

Author: lihaoyi

example/package.mill

@@ -151,8 +151,8 @@ object `package` extends Module {
     val upstreamOpt = upstreamCross(this.moduleSegments.parts.dropRight(1).last)
       .flatMap(_.valuesToModules.get(List(crossValue)))
 
-    def testRepoRoot = Task {
-      os.copy(super.testRepoRoot().path, Task.dest, mergeFolders = true)
+    def testRepoSourceRoot = Task {
+      os.copy(super.testRepoSourceRoot().path, Task.dest, mergeFolders = true)
       for (suffix <- Seq("build.mill")) {
         for (lines <- buildScLines() if os.exists(Task.dest / suffix)) {
           os.write.over(Task.dest / suffix, lines.mkString("\n"))
@@ -177,10 +177,10 @@ object `package` extends Module {
       case None => Task { None }
       case Some(upstream) => Task {
           Some {
-            val upstreamRoot = upstream.testRepoRoot().path
+            val upstreamRoot = upstream.testRepoSourceRoot().path
             val suffix = Seq("build.mill").find(s => os.exists(upstreamRoot / s)).head
-            val upstreamLines = os.read.lines(upstream.testRepoRoot().path / suffix)
-            val lines = os.read.lines(super.testRepoRoot().path / suffix)
+            val upstreamLines = os.read.lines(upstream.testRepoSourceRoot().path / suffix)
+            val lines = os.read.lines(super.testRepoSourceRoot().path / suffix)
 
             import collection.mutable
             val groupedLines = mutable.Map.empty[String, mutable.Buffer[String]]
@@ -242,7 +242,7 @@ object `package` extends Module {
      * Parses a `build.mill` for specific comments and return the split-by-type content
      */
     def parsed: T[Seq[(String, String)]] = Task {
-      mill.testkit.ExampleParser(testRepoRoot().path)
+      mill.testkit.ExampleParser(testRepoSourceRoot().path)
     }
 
     def rendered = Task {

website/package.mill

@@ -16,6 +16,7 @@ object `package` extends mill.Module {
    * API documentation generated with ScalaDoc.
    */
   object apidocs extends UnidocModule {
+    def dummy = Task { PathRef(Task.dest) }
     def unidocDocumentTitle = Task { "Mill" }
 
     def unidocCompileClasspath =
@@ -389,73 +390,74 @@ object `package` extends mill.Module {
     }
   }
   def githubPages: T[PathRef] = Task {
-    generatePages(authorMode = false).apply().apply(oldDocSources().map(_.path))
+    generatePages(false, apidocs.unidocSite).apply().apply(oldDocSources().map(_.path))
   }
 
   def localPages: T[PathRef] = Task {
-    val pages = generatePages(authorMode = true).apply().apply(oldDocSources().map(_.path))
+    val pages = generatePages(true, apidocs.unidocLocal).apply().apply(Nil)
     Task.log.streams.out.println(
       s"You can browse the pages at: ${(pages.path / "index.html").toNIO.toUri()}"
     )
     pages
   }
   def fastPages: T[PathRef] = Task {
-    val pages = generatePages(authorMode = true).apply().apply(Nil)
+    val pages = generatePages(true, apidocs.dummy).apply().apply(Nil)
     Task.log.streams.out.println(
       s"You can browse the pages at: ${(pages.path / "index.html").toNIO.toUri()}"
     )
     pages
   }
 
-  def generatePages(authorMode: Boolean) = Task.Anon { (extraSources: Seq[os.Path]) =>
-    Task.log.streams.err.println("Creating Antora playbook ...")
-    // dependency to sources
-    source()
-    val docSite = Task.dest
-    val playbook = docSite / "antora-playbook.yml"
-    val siteDir = docSite / "site"
-    os.write(
-      target = playbook,
-      data = githubPagesPlaybookText(authorMode).apply().apply(extraSources),
-      createFolders = true
-    )
-    Task.log.streams.err.println("Running Antora ...")
-    runAntora(
-      npmDir = npmBase(),
-      workDir = docSite,
-      args = Seq(
-        playbook.last,
-        "--to-dir",
-        siteDir.toString(),
-        "--attribute",
-        "page-pagination"
-      ) ++
-        Option.when(!authorMode)("--fetch").toSeq
-    )
-    os.write(siteDir / ".nojekyll", "")
-
-    // sanitize devAntora source URLs
-    Task.log.streams.err.println("Sanitizing links ...")
-    sanitizeDevUrls(
-      siteDir,
-      devAntoraSources().path,
-      BuildCtx.workspaceRoot / "docs",
-      mill.api.BuildCtx.workspaceRoot
-    )
+  def generatePages(authorMode: Boolean, apiDocTask: Task[PathRef]) =
+    Task.Anon { (extraSources: Seq[os.Path]) =>
+      Task.log.streams.err.println("Creating Antora playbook ...")
+      // dependency to sources
+      source()
+      val docSite = Task.dest
+      val playbook = docSite / "antora-playbook.yml"
+      val siteDir = docSite / "site"
+      os.write(
+        target = playbook,
+        data = githubPagesPlaybookText(authorMode).apply().apply(extraSources),
+        createFolders = true
+      )
+      Task.log.streams.err.println("Running Antora ...")
+      runAntora(
+        npmDir = npmBase(),
+        workDir = docSite,
+        args = Seq(
+          playbook.last,
+          "--to-dir",
+          siteDir.toString(),
+          "--attribute",
+          "page-pagination"
+        ) ++
+          Option.when(!authorMode)("--fetch").toSeq
+      )
+      os.write(siteDir / ".nojekyll", "")
+
+      // sanitize devAntora source URLs
+      Task.log.streams.err.println("Sanitizing links ...")
+      sanitizeDevUrls(
+        siteDir,
+        devAntoraSources().path,
+        BuildCtx.workspaceRoot / "docs",
+        mill.api.BuildCtx.workspaceRoot
+      )
 
-    for (p <- os.walk(siteDir) if p.ext == "html") {
-      // Manually un-substitute ellipses in normal text to reverse the antora unicode
-      // substitution, because the font for the unicode ellipses looks very bad
-      os.write.over(p, os.read(p).replace("…", "..."))
-    }
+      for (p <- os.walk(siteDir) if p.ext == "html") {
+        // Manually un-substitute ellipses in normal text to reverse the antora unicode
+        // substitution, because the font for the unicode ellipses looks very bad
+        os.write.over(p, os.read(p).replace("…", "..."))
+      }
 
-    // only copy the "api" sub-dir; api docs contains a top-level index.html with we don't want
-    val unidocSrc = if (authorMode) apidocs.unidocLocal().path else apidocs.unidocSite().path
-    Task.log.streams.err.println(s"Copying API docs from ${unidocSrc} ...")
-    os.copy(unidocSrc, siteDir / "api/latest", createFolders = true)
+      // only copy the "api" sub-dir; api docs contains a top-level index.html with we don't want
+      val unidocSrc = apiDocTask().path
+      Task.log.streams.err.println(s"Copying API docs from ${unidocSrc} ...")
+      os.copy(unidocSrc, siteDir / "api/latest", createFolders = true)
 
-    PathRef(siteDir)
-  }
+      PathRef(siteDir)
+    }
 
   def sanitizeDevUrls(
       dir: os.Path,
@@ -500,7 +502,7 @@ object `package` extends mill.Module {
 
   def allLinksAndAnchors
       : T[IndexedSeq[(os.Path, Seq[(String, String)], Seq[(String, String)], Set[String])]] = Task {
-    val base = fastPages().path
+    val base = localPages().path
     val validExtensions = Set("html", "scala")
     for (path <- os.walk(base) if validExtensions(path.ext))
       yield {