Properly cache apps_paths configuration, closes #5622

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/mix/lib/mix/project.ex

@@ -233,9 +233,10 @@ defmodule Mix.Project do
   @spec apps_paths() :: %{atom => Path.t} | nil
   def apps_paths(config \\ config()) do
     if apps_path = config[:apps_path] do
-      Mix.ProjectStack.read_cache(:apps_path) ||
-        Mix.ProjectStack.write_cache(:apps_path,
-          config[:apps] |> umbrella_apps(apps_path) |> to_apps_path(apps_path))
+      key = {:apps_paths, Mix.Project.get!}
+      Mix.ProjectStack.read_cache(key) ||
+        Mix.ProjectStack.write_cache(key,
+          config[:apps] |> umbrella_apps(apps_path) |> to_apps_paths(apps_path))
     end
   end
 
@@ -249,7 +250,7 @@ defmodule Mix.Project do
     apps
   end
 
-  defp to_apps_path(apps, apps_path) do
+  defp to_apps_paths(apps, apps_path) do
     for app <- apps,
         path = path_with_mix_exs_otherwise_warn(app, apps_path),
         do: {app, path},

lib/mix/lib/mix/project_stack.ex

@@ -18,7 +18,7 @@ defmodule Mix.ProjectStack do
     get_and_update fn %{stack: stack} = state ->
       # Consider the first children to always have io_done
       # because we don't need to print anything unless another
-      # project talks ahold of the shell.
+      # project takes ahold of the shell.
       io_done? = stack == []
 
       config  = Keyword.merge(config, state.post_config)