Package loader: Uniformize author and message

Authors: use the empty string '' instead of placeholders
Message: use the same message format (inspired by the Debian loader)
for all loaders, instead of the empty string / the version /
something else; except for PyPI and Deposit (which have a better
format because we have more metadata available).

Additionally, this commit adds test of each release object,
instead of only relying on its hash.