Added testing; corrected parsed_versions_by_package
Added testing; corrected parsed_versions_by_package

 - Added assertion testing after each processed sdist
 - Corrected logic for parsed_versions_by_package to capture the
version at the beginning of processing, not at the end (so even if
metadata capture fails, we still have the version in the versions list,
as intended)
 - renamed parsed_versions_by_package to versions_by_package and
dependencies_by_package to dependencies_by_package_version (still not
the best name, but better?)

awwad/depresolve