Organize modules consistently

[graebm]

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Currently the Transfer Manager is kinda organizing things like the SDK does (e.g. putting a lot of stuff under types:: ) but it's not 100% consistent.

We should either:

1. Strive for consistency with the SDK
   • e.g. types::UploadChecksumStrategy
2. Organize by "system"
   • e.g. operation::upload::ChecksumStrategy