Chunking Refactor Step 2 (#6120)
### Description
The second step in our chunking refactoring, this removes our use of
`Module::as_chunk` and `Module::as_root_chunk`. Instead, the only way to
generate a chunk is directly from a root `ChunkItem`.
Re: https://github.com/vercel/turbo/pull/6104
### Testing Instructions
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Closes WEB-1720
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Co-authored-by: Tobias Koppers <tobias.koppers@googlemail.com>
