fix: Backfill missing pnpm workspace importer entries during prune (#12514)
## Summary
- `turbo prune` now backfills empty importer entries for pnpm workspace
packages that have no dependencies
## Problem
When a pnpm workspace package has zero dependencies, its lockfile may
omit the importer entry entirely. After `turbo prune`, the pruned
lockfile would also lack the entry, causing `pnpm install
--frozen-lockfile` to fail because pnpm expects every workspace package
to have a corresponding importer entry.