This change uses the aws cli, if available, to retrieve spec files
from the mirror to a local temp directory, then parallelizes the
reading of those files from disk using multiprocessing.ThreadPool.
If the aws cli is not available, then a ThreadPool is used to fetch
and read the spec files from the mirror.
Using aws cli results in ~16 times speed up to recreate the binary
mirror index, while just parallelizing the fetching and reading
results in ~3 speed up.
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