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.
Break up URL into base part and relative path. Match checksum against relative path
and never against full URL.
This might be fixing security issue if aptly was incorrectly matching against
wrong part of Release file.
* 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
