cmd/go/internal/work: cache cgo invocations for vet, build modes
Even if we had an up-to-date package binary, we reran cgo anyway if
(1) we needed a header file for buildmode c-archive or c-shared, or
(2) we needed cgo-translated files source files for input to go vet.
Cache those outputs too, so that we can avoid cgo if possible.
Working toward exposing the cgo-generated files in go list.