Read 'phabricator.uri' and 'default' out of more config sources

Summary: We have old/inconsistent code for reading these. Instead, read from sources in a modern way.

Test Plan:
  - Removed "default" and "phabricator.uri" from user/local/project config.
  - Set "default" in "/etc/arcconfig".
  - Ran `arc tasks` and got a "connect to Phabricator" message.

Reviewers: btrahan, davedash

Reviewed By: davedash

Subscribers: davedash, epriestley

Differential Revision: https://secure.phabricator.com/D9105

scripts/arcanist.php

@@ -204,14 +204,14 @@
   if ($force_conduit) {
     $conduit_uri = $force_conduit;
   } else {
-    $project_conduit_uri = $configuration_manager->getProjectConfig(
+    $conduit_uri = $configuration_manager->getConfigFromAnySource(
       'phabricator.uri');
-    if ($project_conduit_uri) {
-      $conduit_uri = $project_conduit_uri;
-    } else {
-      $conduit_uri = idx($global_config, 'default');
+    if ($conduit_uri === null) {
+      $conduit_uri = $configuration_manager->getConfigFromAnySource(
+        'default');
     }
   }
+
   if ($conduit_uri) {
     // Set the URI path to '/api/'. TODO: Originally, I contemplated letting
     // you deploy Phabricator somewhere other than the domain root, but ended