Refactor DownloadQueue handling

- Use undocumented (for now) `HOMEBREW_DOWNLOAD_CONCURRENCY` instead
  of `--concurrency` flag and avoid passing around `concurrency`
- Create and use `Formula#enqueue_resources_and_patches` helper method
- Rename some method calls to be more obvious
- Use `Downloadable` type to simplify type checks
- General refactoring