Fix Phage unit test issue with ErrorTrap

Summary: The Phage agent build became dependent on PhutilErrorTrap after its introduction. Rather than build it into the agent, make the dependency soft. Also make the ServiceProfiler dependency soft to limit the surface area of the agent.

Test Plan: `arc unit --everything`

Reviewers: btrahan, wez, michael.plusnin

Reviewed By: wez

CC: aran

Differential Revision: https://secure.phabricator.com/D6792