Hash aliasing support

[unreadablewxy]

There's been cases where crawlers drag in files that looks the same and definitely are the same but hashes slightly differently either due to re-encoding or whatever other transformative processes.

• We should be able to easily build up a list of known hash aliases by creating hard links into say collection/by-id-alias where files are named based on alais_size.alias_hash.extension
• Would require a new conflict resolution option file = alias HASH|GROUP+INDEX
• Would require a new test at import time against known aliases