cmd/go/internal/cacheprog: drop redundant Prog prefixes
Now that these types are in their own package, drop the unnecessary
Prog prefixes from everything.
Updates #71032
Updates #59719
Change-Id: Id54edf0473754e3b21a71beb72803fb5481206c1
Reviewed-on: https://go-review.googlesource.com/c/go/+/638996
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Austin Clements <austin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>