cmd/vet: cache vet results

[jwebb]

go vet on a large codebase can take a nontrivial amount of time. It seems the Vet result for each package should be cacheable in much the same way as tests, i.e. the cache key is a combination of BuildID and Vet command line flags.

[seankhliao]

cc @matloob

[timothy-king]

You will also need to know if the vettool argument updated. This could be something like taking the hash of the binary at /path/to/tool when given -vettool=/path/to/tool.

[mvdan]

@timothy-king if we can assume that the vettool program is in Go, I imagine we could use something akin to go tool buildid to obtain the hash embedded in the binary. It doesn't make a big difference in terms of design, just an FYI.

[ianlancetaylor]

This is an optimization that is not visible to users, so taking this out of the proposal process.