now we no longer go through temporary files when names don't change,

now we no longer go through temporary files when names don't change,
make the matching algorithm between the old and the new package more
precise: keep a list of matching checksums, instead of just one candidate.

For starters, empty files will always yield the same checksum, but it seems
some software (python for instance) loves installing multiple copies of
the same file.  This does prevent 500 "name mismatches" out of 2500 in
python-3.9, for instance.