Include compile classpath in unidoc module (#5858)

Solves generating unidoc in
#5836, which otherwise fails
because we don't properly traverse the
`compileMvnDeps`/`compileModuleDeps` of the transitive module graph,
which can result in some necessary parts of the classpath missing from
the unidoc compile classpath

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

Author: lihaoyi

libs/scalalib/src/mill/scalalib/UnidocModule.scala

@@ -18,7 +18,9 @@ trait UnidocModule extends ScalaModule {
   def unidocVersion: T[Option[String]] = None
 
   def unidocCompileClasspath: T[Seq[PathRef]] = Task {
-    Seq(compile().classes) ++ Task.traverse(moduleDeps)(_.compileClasspath)().flatten
+    Seq(
+      compile().classes
+    ) ++ Task.traverse(transitiveModuleCompileModuleDeps)(_.compileClasspath)().flatten
   }
 
   /**