cmd/go: change Printer.Output -> Printer.Printf for consistency

cmd/go: change Printer.Output -> Printer.Printf for consistency

Currently, the Printer interface has `Output`, which acts like Print
and `Errorf`, which acts like Printf. It's confusing that the
formatting style is tied to whether it's regular output or an error.

Fix this by replacing Output with Printf, so both use Printf-style
formatting.

Change-Id: I4c76f941e956f2599c5620b455bf41e21636b44e
Reviewed-on: https://go-review.googlesource.com/c/go/+/627795
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>