Raise early if mix xref is used at the umbrella root

None of the commands worked as desired and
the only support for umbrellas is in the now
removed compiler callback.

Closes #12629.

Author: josevalim

lib/mix/lib/mix/tasks/xref.ex

@@ -5,7 +5,6 @@ defmodule Mix.Tasks.Xref do
     only: [read_manifest: 1, source: 0, source: 1, source: 2, module: 1]
 
   @shortdoc "Prints cross reference information"
-  @recursive true
   @manifest "compile.elixir"
 
   @moduledoc """
@@ -256,7 +255,8 @@ defmodule Mix.Tasks.Xref do
 
     * `--include-siblings` - includes dependencies that have `:in_umbrella` set
       to true in the current project in the reports. This can be used to find
-      callers or to analyze graphs between projects
+      callers or to analyze graphs between projects (it applies only to `trace`
+      subcommand)
 
     * `--no-compile` - does not compile even if files require compilation
 
@@ -289,6 +289,12 @@ defmodule Mix.Tasks.Xref do
 
   @impl true
   def run(args) do
+    if Mix.Project.umbrella?() do
+      Mix.raise(
+        "mix xref is not supported in the umbrella root. Please run it inside the umbrella applications instead"
+      )
+    end
+
     Mix.Task.run("compile", args)
     Mix.Task.reenable("xref")