Silence superfluous warnings from including nonexistent __init__.php and source files

Summary:
We double-emit warnings here (PHP emits some directly to stdout/stderr, then we
throw an exception). We don't want to use "@" because it will keep us from
getting parse errors, so just disable warnings instead.

Critically, 'arc liberate' can not generate __init__.php files cleanly without
this since the host process fails ("unexpected data on stderr") when the library
mapper emits a bunch of warnings. It still works right now but you have to run
it twice.

Test Plan:
Ran 'arc liberate' on library with missing __init__.php files.

Reviewed By: tuomaspelkonen
Reviewers: aran, tuomaspelkonen, jungejason
CC: aran, tuomaspelkonen
Differential Revision: 393