output: use strings instead of PathInfo for performance reasons

[pmrowla]

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Partial fix for #3635.

[pmrowla]

The optimization in OutputBase._collect_used_dir_cache improves the _cloud_status runtime from 938s to 337s in cProfile, for an S3 remote with 2M files.

There are still several places we can improve in RemoteBASE and RemoteLOCAL, since remote.checksum_to_path_info is slow when dealing with large numbers of files. The issue here would be that it looks like it may end up being a fairly substantial refactoring, since optimizing where pathinfo is currently used in RemoteLOCAL.status ends up affecting several other places in the upload/download pipeline which currently expect pathinfos. In testing, I was able to get _cloud_status runtime down to 221s (but with broken push/pull).

@efiop, should I continue with this or do we want to just leave it at only the used_dir_cache change for now?

[efiop]

@pmrowla Your call. I would be perfectly happy to merge this first though. Astonishing results!

[efiop]

@pmrowla About checksum_to_path_info. We used to have checksum_to_path not that long ago, so maybe we need to go back and just convert to PathInfo where we really need it. Or we could have both checksum_to_path and checksum_to_path_info and use where appropriate, not sure.

[pmrowla]

@efiop in that case, once I fix whatever is failing CI in windows I'll mark this PR as reviewable/mergeable. We can leave the original issue open and I'll keep looking into optimizing checksum_to_path_info

[efiop]

Thanks @pmrowla !