chore: ensure that all packages have comments

[G-Rath]

Branched off #546, and will conflict once that has been landed

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This mirrors an internal linter, though this linter requires that the package comment be in a file that is not ignored e.g. due to go:build conditions, meaning it considers a few packages as missing comments as they're in a Windows-only file.

Currently, I've addressed that by copying the comment into a non-Windows file without removing the previous comment, though we might want to discuss what convention we actually follow - I think there's at least two possible ones to consider:

1. always put the comment in all files in a go:build "set" i.e. xyz_dummy.go and xyz_windows.go (downside being we're having to maintain the comment twice)
2. always put the package comment in a file that is never ignored when possible (possible downside is that often that can be a file holding utilities rather than the actual meat of the package, and if there isn't such a file then we'd have to do 1. anyway)

Relates to #455
Relates to #274