Turbopack: make available_modules an OperationVc and simplify merged module bookkeeping

[sokra]

What?

Reworks how chunk_group_content exposes available_modules for downstream chunking, and consolidates the bookkeeping for merged modules in ChunkGroup.

• Splits ChunkGroupContent into a turbo_tasks::value ChunkGroupContentInner (carrying chunkable_items, batch_groups, async_modules, traced_modules, and available_modules) plus a thin wrapper that pairs it with the resulting AvailabilityInfo. chunk_group_content_operation now runs entirely inside turbo_tasks, allowing AvailableModules to store its set as an OperationVc<AvailableModulesSet> and .connect() it on read.
• Replaces the separate included set and should_create_chunk_item_for method on MergedModuleInfo with an Option<ChunkableModule> value type on the replacements map. Call sites now decide all three cases (replace / skip / keep) in one match via should_replace_module.

Why?

• AvailableModules::snapshot did not correctly derive its invalidation priority, which caused double invalidations: the snapshot was treated as a low-priority dependency even though it gated chunking work, so when its inputs changed turbo-tasks would invalidate it after dependent computations had already started, triggering them a second time. Exposing available_modules as an OperationVc makes AvailableModules inherit the correct invalidation priority from the operation that produces it, which removes the double-invalidation.
• The previous MergedModuleInfo API had two parallel data structures (replacements and included) and a helper that combined them. Folding included into the value of replacements removes the duplication, makes the three states (replace, skip-because-merged, keep) explicit, and shrinks the call-site logic in chunk_group.rs.

How?

available_modules as an OperationVc:

• New ChunkGroupContentInner is a turbo_tasks::value holding the chunkable items, batch groups, async/traced modules, and an OperationVc<AvailableModulesSet> for available_modules.
• ChunkGroupContent becomes a plain struct wrapping ResolvedVc<ChunkGroupContentInner> together with the AvailabilityInfo computed for the group.
• chunk_group_content_operation is an OperationVc-producing function so its inner available_modules can be returned as an OperationVc<AvailableModulesSet>. AvailableModules now stores that operation Vc and .connect()s it whenever the set is read, so consumers reach the set through the operation and inherit its priority.
• Browser and Node.js chunking contexts are updated to thread the new shape through make_chunk_group and friends.

MergedModuleInfo simplification:

• replacements changes from HashMap<ChunkableModule, ChunkableModule> to HashMap<ChunkableModule, Option<ChunkableModule>>: Some(m) means "replace with m", None means "skip — already included in another merged module", absent means "keep as-is".
• The old included set and should_create_chunk_item_for helper are removed; should_replace_module returns the three-way outcome and consumers match on it once.
• Call sites in chunk_group.rs are restructured around the single match so the logic is no longer split across two lookups.

Notes

• No user-facing behavior change is intended — this is an internal refactor in turbopack-core (chunk_group_content, available_modules, chunk_group) and the browser/nodejs chunking contexts.
• Existing chunking tests in turbopack-core exercise the new shape.

[github-actions]

Tests Passed

Commit: 7ae6320