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.

Change-Id: I339ecace925d2b0adc235a17977ecadb3d636c73
Reviewed-on: https://go-review.googlesource.com/108015
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

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