Run this command across the source:
perl -pi.bak -e 's/(20[012][0-9])-20[12][012]/$1-2023/g' `git ls-files`
and remove changes to po{,-docs}/*.po{,t} (these will be regenerated
later when we run 'make dist').
Isolate the logic for the memoized disk cache in a small module, so it
can be reused for other tools.
Other than refactoring, there should be no behaviour changes.
