cmd/go: move GOCACHEPROG protocol types to their own package
This is a step toward making it easy to point to them in
documentation. The other option is that we copy-paste all of these
type definitions wholesale, which seems ridiculous.
Updates #71032
Updates #59719
Change-Id: I7117e03308ae0adc721ed7a57792c33ba68ce827
Reviewed-on: https://go-review.googlesource.com/c/go/+/638995
Auto-Submit: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>