Apply retries as global, config-level option `downloadRetries` so that
it can be applied to any aptly command which downloads objects.
Unwrap `errors.Wrap` which is used in downloader.
Unwrap `*url.Error` which should be the actual error returned from the
HTTP client, catch more cases, be more specific around failures.
There are two fixes here:
1. Abort package download immediately as ^C is pressed.
2. Import all the already downloaded files into package pool,
so that next time mirror is updated, aptly won't download them
once again.
* Drop multi-threaded downloader. It doesn't really belong here -
some places require it, some do not, but it's definitely not the
right place to handle it, as it's being used only when updating
mirrors
* Pass expectedChecksums as pointer, so it's easy to drive `nil` value,
and also downloader can fill back checksums (not implemented right now).
* Break down downloader and tests into more files
* Use pkg/errors instead of fmt
