cmd/go: reject list when -find and -export are used together

cmd/go: reject list when -find and -export are used together

Fixes #51952.

Change-Id: If2cfc41d65373ca38cfb7b0396be8988d444eb5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/396074
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>