Do not clean whole build on clean, only libs

Author: José Valim

lib/mix/lib/mix/tasks/clean.ex

@@ -25,7 +25,7 @@ defmodule Mix.Tasks.Clean do
 
     if opts[:all] do
       Mix.Task.run("deps.clean", args)
-      File.rm_rf(Path.dirname(Mix.Project.build_path))
+      File.rm_rf(Path.dirname(Mix.Project.app_path))
     else
       File.rm_rf(Mix.Project.app_path)
     end

lib/mix/test/mix/tasks/clean_test.exs

@@ -55,7 +55,8 @@ defmodule Mix.Tasks.CleanTest do
       assert File.exists?("_build/shared/lib/ok")
       Mix.Tasks.Clean.run ["--all"]
 
-      refute File.exists?("_build")
+      assert File.exists?("_build/shared")
+      refute File.exists?("_build/shared/lib")
       assert_received { :mix_shell, :info, ["* Cleaning ok"] }
       assert_received { :mix_shell, :info, ["* Cleaning unknown"] }
     end