This fixes the weird/broken existing behaviour which was incorrectly
creating symlinks at download time. It also defers much more logic to
the original code.
For clarity, rename the existing `API::Download` class to
`API::SourceDownload`.
While we're here:
- add a/improve the `download_type` method on all `Downloadable`
subclasses to improve download queue output format
- move some logic to `RetryDownload`
This came up in the AGM and has bothered me for years: let's actually
split out `software_spec.rb` into one file per class, as is more typical
in Ruby.
This will make these classes easier to find.
