Modernize bin/files migrate and fix behavior on chunk engines

Summary: Ref T9828. Mostly just does a minor modernization pass, but also doesn't migrate chunked files since it's not meaningful (they don't have data, directly).

Test Plan: Ran bin/files migrate with various flags. Migrated S3 -> Blob and Blob -> S3.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9828

Differential Revision: https://secure.phabricator.com/D14981