Improve binary-compatible Task overriding (in future Mill versions)

[lefou]

When overriding tasks in derived traits, we often have the issue, that the change is not binary compatible when it also references the super-task.

The most prominent case is, when an already overridden task needs to reference it's super-task, but previously didn't.

For example, let's assume, we have a task ScalaModule.allSourceFiles and instead of directly listing all source files, we instead just want to add all Scala files to the already evaluated source files (from JavaModule).

   override def allSourceFiles: T[Seq[PathRef]] = Task {
-    Lib.findSourceFiles(allSources(), Seq("scala", "java")).map(PathRef(_))
+    super.allSourceFiles() ++ Lib.findSourceFiles(allSources(), Seq("scala")).map(PathRef(_))
   }

The corresponding MiMa report might look like this:

* abstract synthetic method mill$scalalib$ScalaModule$$super$allSourceFiles()mill.api.Task#Simple in interface mill.scalalib.ScalaModule is present only in current version
  filter with: ProblemFilter.exclude[ReversedMissingMethodProblem]("mill.scalalib.ScalaModule.mill$scalalib$ScalaModule$$super$allSourceFiles")

This makes it really hard to evolve Mills trait-based modules in a binary compatible way.

A way we could avoid this issue would be to have the super-call to begin with, even if we don't need the super-result right now.

   override def allSourceFiles: T[Seq[PathRef]] = Task {
+    val _ = super.findSourceFiles
     Lib.findSourceFiles(allSources(), Seq("scala", "java")).map(PathRef(_))
   }

Since we already use a macro to generate the effective definition, it should be easy to generate a reference to the super-class as part of the macro implementation.

While this would solve the issue of changing existing tasks, we still can't add new overrides to existing task with a reference to the super-task without breaking binary compatibility.
It would be nice, if we could somehow generate super-references for all potential tasks in each trait that extends Module. This is probably only possible with an compiler plugin.