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