The iotuil package has been deprecated since go1.16; the various
functions it provided now exist in the os and io packages. Replace all
instances of ioutil with either io or os, as appropriate.
Completely refactor the way we organize our code related to OCSP.
- Move it all into one `//ocsp/` package, rather than having multiple
top-level packages.
- Merge the OCSP updater's config sub-package with its parent
(since it isn't necessary to break it out to avoid cyclic imports).
- Remove all `Source` logic from ocsp-responder's `main.go`, because
it was difficult to mentally trace the control flow there.
- Replace that logic with a set of composable `Source`s in the
`//ocsp/responder/` package, each of which is good at just one thing.
- Update the way the filters work to make sure that the request's
`IssuerKeyHash` and the response's `ResponderName` can both
be derived from the same issuer certificate, ensuring that the req and
resp are correctly matched.
- Split the metrics into a separate metric for each `Source`, so we can
tell what all of them are doing, not just aggregate behavior.
- Split the tests into individual files for each `Source`, and update them
for the new public interfaces.
Aaron Gable