turbopack: Chunking Refactor Step 2 (#56504)
### What?
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.
### Why?
In the end we want to avoid creating chunks from modules directly, but enforce everything going through the ChunkingContext to be chunked. This allows us to replace the existing chunking algorithm with a much more efficient one that avoid duplication between chunks in first place and doesn't require a post-chunking optimization.
### How?
https://github.com/vercel/turbo/pull/6120
Re: https://github.com/vercel/next.js/pull/56467
Closes WEB-1721
Co-authored-by: Tobias Koppers <1365881+sokra@users.noreply.github.com>
