cmd/go: recommend -d when -m is used

In #32038, it was decided to remove get's -m, since one former use case
is removed, and the other can be done via -d, as pointed by Russ.

However, a user getting this short error might not realise that they can
switch to -d to skip building packages. Add a short mention to point
them in the right direction.

It's important to note "packages", because -m was a flag that acted on
modules, while -d acts on packages. Simply replacing -m with -d might
not be enough in some cases because of that distinction.

Change-Id: I0947b25c4223bdad3cd0e535848527da8db8a16d
Reviewed-on: https://go-review.googlesource.com/c/go/+/179361
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: mvdan

src/cmd/go/internal/modget/get.go

@@ -258,7 +258,7 @@ func runGet(cmd *base.Command, args []string) {
 		fmt.Fprintf(os.Stderr, "go get: -fix flag is a no-op when using modules\n")
 	}
 	if *getM {
-		base.Fatalf("go get: -m flag is no longer supported")
+		base.Fatalf("go get: -m flag is no longer supported; consider -d to skip building packages")
 	}
 	modload.LoadTests = *getT