Implement support for source archives

There are a number of scenarios where it would be useful to be able to get license information for a source archive.

There are a few forms this could take:
* Point at an internet-accessible address for the source archive (this would be required for crawling)
* Look up definitions based on a file hash of the source archiving

Crawling would be relatively simple because it would basically be:
1. Fetch the archive
2. Extract the archive
3. Run the standard tools over the contents of the archive