simplify: remove custom locking, parallelize all entry types

Address round 2 review feedback from @MikeMcQuaid:

- Remove all custom FormulaLock-based locking from ParallelInstaller.
  Rely on `brew install`'s built-in keg locking instead, with the
  dependency map preventing same-tree parallel installs.
- Extend parallel installation to all entry types (casks, taps,
  extensions), not just formulae. Non-Brew entries have no inter-entry
  dependencies and can all run in parallel immediately.
- Simplify build_dependency_map to handle both Brew (formula deps)
  and non-Brew (empty deps) entries in a single pass.

Net -82 lines. Typecheck, style, and tests pass.