Store and retrieve on demand scanned packages/archives

[AyanSinhaMahapatra]

From a discussion with @pombredanne

Packages which are downloaded and scanned in SCIO can be optionally
stored and accessed to have a copy of the packages which are being
used for a specific product for reference and future use, and could
be used to meet source redistribution obligations too.

For this we have to store all packages/archives which are downloaded,
create an API and index by URL/checksum to get these packages on-demand,
store metadata/history and logs for these downloaded/stored packages,
and also maybe support the ability to add external storage options.

There should be configuration variable to turn this on to enable these
features, and connect external databases/storage.