Dead symbol detector should trace re-exports through barrel files

[fixcik]

Problem

When a symbol is re-exported through a barrel file (index.ts), the dead_code detector considers the originating file as "alive" because it is re-exported by an entry point. However, the re-exported symbol itself may never be imported by any consumer.

This means symbols that are only used in tests but re-exported through barrel files are invisible to both:

• dead_code detector (file is re-exported → not dead)
• count_test_imports: false option (file-level check bypassed by re-export)

Example

packages/utils/src/
  index.ts          ← entry point, barrel: export * from './payload-sanitizer.js'
  payload-sanitizer.ts  ← exports sanitizePayload()
  payload-sanitizer.spec.ts ← only consumer of sanitizePayload()

sanitizePayload is never imported by any production code, only by the test file. But the dead_code detector sees that payload-sanitizer.ts is re-exported through index.ts (an entry point) and considers it alive.

Current Behavior

• dead_code detector works at file level
• is_reexported() returns true because index.ts re-exports payload-sanitizer.ts
• File is not flagged, even though no consumer ever imports sanitizePayload from the barrel

Expected Behavior

The dead symbol detector (dead_symbols) should trace re-exports through barrel files and verify that the re-exported symbol is actually imported by at least one consumer downstream. If no consumer imports the symbol (only tests do), it should be flagged.

Possible Approaches

1. Enhance dead_symbols detector — when a symbol is re-exported through a barrel, check if any file imports that symbol from the barrel (not just that the barrel re-exports it)
2. Transitive usage analysis — follow re-export chains and verify end-to-end usage
3. Combine with count_test_imports — if the only downstream consumers are test files, flag the symbol

Related

• Dead code detector should flag code used only in tests #35 — count_test_imports option (file-level, doesn't cover barrel re-exports)