Since we are going to switch to releases, this looks like the right
time to make some changes in the objects written by package loaders
to make them consistent with each other.
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.