Chunking Refactor Step 1 (#56467)
### What?
This change moves a few methods around.
Module::as_chunk is moved to ChunkItem::as_chunk as temporary intermediate state.
EcmascriptPlaceable::as_chunk_item is moved to ChunkableModule::as_chunk_item. This generalizes the concept of converting a Module into a ChunkItem
### Why?
This is the first step of refactoring the chunking. 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?
see https://github.com/vercel/turbo/pull/6104
Closes WEB-1715
Co-authored-by: Justin Ridgewell <112982+jridgewell@users.noreply.github.com>
