cc @bcmills @jayconrod

It's possible under some rare circumstances (a system crash or loss of power during a write) for corrupt files to be written to the module cache. We should add commands to help clear away these corrupt files if they exist so that those serving a GOPROXY from their module cache can have more certainty that they're serving good data.

Two potential commands we discussed are
(1) go clean for a single module, to remove modules suspected of being corrupted
(2) go mod verify -clean to remove module versions that fail checksums