buildMix: default to removing target config (#430275)

pkgs/development/beam-modules/build-mix.nix

@@ -25,12 +25,15 @@
   meta ? { },
   enableDebugInfo ? false,
   mixEnv ? "prod",
+  removeConfig ? true,
   # A config directory that is considered for all the dependencies of an app, typically in $src/config/
   # This was initially added, as some of Mobilizon's dependencies need to access the config at build time.
   appConfigPath ? null,
   ...
 }@attrs:
 
+assert appConfigPath != null -> removeConfig;
+
 let
   shell =
     drv:
@@ -79,10 +82,17 @@ let
             runHook preConfigure
 
             ${./mix-configure-hook.sh}
+            ${lib.optionalString (removeConfig && isNull appConfigPath)
+              # By default, we don't want to include whatever config a dependency brings; per
+              # https://hexdocs.pm/elixir/main/Config.html, config is application specific.
+              ''
+                rm -rf config
+                mkdir config
+              ''
+            }
             ${lib.optionalString (!isNull appConfigPath)
-              # Due to https://hexdocs.pm/elixir/main/Config.html the config directory
-              # of a library seems to be not considered, as config is always
-              # application specific. So we can safely delete it.
+              # Some more tightly-coupled dependencies do depend on the config of the application
+              # they're being built for.
               ''
                 rm -rf config
                 cp -r ${appConfigPath} config