Allowing using `HOMEBREW_DOWNLOAD_CONCURRENCY` to use the
`DownloadQueue` for `brew install` by downloading and extracting
bottles in parallel.
This requires some fixes in e.g. `Dependency` and `FormulaInstaller`
to be able to front-load all downloads and handle parallelisation of
bottle pouring.
Behaviour without `HOMEBREW_DOWNLOAD_CONCURRENCY` set should be
unchanged.
Attestations are not handled for now and the UI should be improved
before we roll this out to users.
Post-install upgrades are not yet parallelised.
Co-authored-by: Carlo Cabrera <github@carlo.cab>
