cmd/go: abstract build cache, support implementations via child process

cmd/go: abstract build cache, support implementations via child process

Via setting GOCACHEPROG to a binary which speaks JSON over
stdin/stdout.

For now, it requires GOEXPERIMENT=cacheprog.

Fixes golang/go#59719

Change-Id: I824ff04d5ebdf0ba4d1b5bc2e9fbaee26d34c80f
Reviewed-on: https://go-review.googlesource.com/c/go/+/486715
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>