Add a global config option to restrict root creation

Summary:
We've found in practice that it's too easy for relatively careless clients to unintentionally open nested watches. To fix this, add an optional configuration option called `root_restrict_files` that
restricts watch roots to directories in which certain files are present.

Test Plan: `arc unit --everything`.

Reviewers: wez

Differential Revision: https://phabricator.fb.com/D898432

Task ID: 2647765

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extra : rebase_source : 92274a5c1512f3c25407d4cbddb4aa3519c248b9
extra : amend_source : f0b0d0c4d6fd76f767fe531189c0707e5d679f85