Simplify create_manifest_data

[Rich-Harris]

Started work on #4275, but found create_manifest_data quite difficult to work with because of some unnecessary complexity. This puts us on better footing by eliminating that complexity.

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[benmccann]

I didn't really look at the changes here. any chance this might fix #4294 ?

[Rich-Harris]

I don't think so. Will need to take a closer look at that issue

[Rich-Harris]

It does not. Fixing that issue will involve some fundamental changes to how trailingSlash is handled