cmd/go: add go mod why

A very common question is
"why is this package or module being kept
by go mod vendor or go mod tidy?"

go mod why answers that question.

Fixes #26620.

Change-Id: Iac3b6bbdf703b4784f5eed8e0f69d41325bc6d7f
Reviewed-on: https://go-review.googlesource.com/128359


Reviewed-by: Bryan C. Mills <bcmills@google.com>