- 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
This is the first step towards using the download queue for all
download operations e.g. `brew install`, `brew upgrade`,
`brew reinstall` etc.
While we're here, do some API cleanup and Sorbet type improvements.
