All package loaders but deposit had logic to compute some object
from the new packageinfo, some other objects from the known artifacts,
and compare them.
This commit moves the comparison logic to the base class, and unifies
the two computation interfaces, respectively as an extid() method
on TPackageInfo and a method on the loader.
This unified object for comparison is a byte string,
which is internal to each loader for now, but a future commit
will read and write it from/to the ExtID storage instead of
computing it from the 'original_artifacts' present in
revision metadata.