Fix an issue where "arc" may fail on startup when trying to read older "default" config

Summary:
See PHI1663. If "phabricator.uri" is not configured, we try to fall back to "default", but doesn't have a modern config specification and fails.

Instead, read "default" as a raw config value to preserve compatible behavior. My intent is to eventually remove it.

Test Plan:
In a directory with no "phabricator.uri" config available, ran echo {} | arc call-conduit conduit.ping. After the patch, got a reasonable error instead of a fatal.

In a directory with "default" configured but not "phabricator.uri", ran the same command. After the patch, got a ping.

Maniphest Tasks: T13497

Differential Revision: https://secure.phabricator.com/D21039