Detect library inclusion conflicts in libphutil

Summary:
This is one half of making the 'arc' error message for running arc against
itself better. Detect the error in libphutil and throw a meaningful exception.

Test Plan:
Ran Phabricator and Arcanist without issues.

Reviewed By: jungejason
Reviewers: jungejason, tuomaspelkonen, aran
CC: aran, jungejason
Differential Revision: 311