Make the behavior of "--load-phutil-library" more explicit

Summary:
This flag disables normal libraries to make it easier to test library changes. Print that out explicitly.

Also update `.arcconfig` to use the modern configuration format.

Test Plan: Ran `arc list --trace`, `arc list --load-phutil-library=src --trace`.

Reviewers: zeeg, btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5768

.arcconfig

@@ -3,7 +3,7 @@
   "conduit_uri" : "https://secure.phabricator.com/",
   "lint.engine" : "PhutilLintEngine",
   "unit.engine" : "PhutilUnitTestEngine",
-  "phutil_libraries" : {
-    "arcanist" : "src/"
-  }
+  "load" : [
+    "src/"
+  ]
 }

scripts/arcanist.php

@@ -96,6 +96,9 @@ try {
   // is configured. This is basically a debugging feature to let you force
   // specific libraries to load regardless of the state of the world.
   if ($load) {
+    $console->writeLog(
+      "Using '--load-phutil-library' flag, configuration will be ignored ".
+      "and configured libraries will not be loaded."."\n");
     // Load the flag libraries. These must load, since the user specified them
     // explicitly.
     arcanist_load_libraries(